Ultrasound imaging with sparse array probes

ABSTRACT

Sparse arrays of transducer elements may be beneficial in providing ultrasound transducer probes with a wide total aperture while containing a manageable number of transducer elements. Sparse arrays made with bulk piezoelectric materials or with arrays of micro-elements can be effectively with ping-based multiple aperture ultrasound imaging techniques to perform real-time volumetric imaging.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 62/287,694, filed on Jan. 27, 2016, titled “UltrasoundImaging Using Apparent Point-Source Transmit Transducer”, and U.S.Provisional Patent Application No. 62/310,482, filed on Mar. 18, 2016,titled “Real-Time Three-Dimensional Ultrasound Imaging” both of whichare herein incorporated by reference in their entirety.

INCORPORATION BY REFERENCE

All publications and patent applications mentioned in this specificationare herein incorporated by reference in their entirety to the sameextent as if each individual publication or patent application wasspecifically and individually indicated to be incorporated by reference.

FIELD

This application relates generally to the field of ultrasound imaging,and more particularly to ping-based ultrasound imaging using sparsearrays of ultrasound transducer elements.

BACKGROUND

In conventional scanline-based ultrasonic imaging, a focused beam ofultrasound energy is transmitted into body tissues to be examined andechoes returned along the same line are detected and plotted to form aportion of an image along the scanline. A complete image may be formedby repeating the process and combining image portions along a series ofscanlines within a scan plane. Any information in between successivescanlines must be estimated by interpolation.

The same process has been extended to obtaining ultrasonic images ofthree-dimensional volumes by combining images from multiple adjacentslices (where each slice is in a different scan plane). Again, anyinformation from any space in between successive scan planes must beestimated by interpolation. Because time elapses between capturingcomplete 2D (two-dimensional) slices, obtaining 3D (three-dimensional)image data for a moving object may be significantly impaired. So-called“4D” (four-dimensional) imaging systems (in which the fourth dimensionis time) strive to produce moving images (i.e., video) of 3D volumetricspace. Scanline-based imaging systems also have an inherent frame-ratelimitation which creates difficulties when attempting 4D imaging on amoving object.

As a result of these and other factors, some of the limitations ofexisting 2D and 3D ultrasonic imaging systems and methods include poortemporal and spatial resolution, imaging depth, speckle noise, poorlateral resolution, obscured tissues and other such problems.

Significant improvements have been made in the field of ultrasoundimaging with the creation of multiple aperture imaging, examples ofwhich are shown and described in Applicant's prior patents andapplications referenced above. Multiple aperture imaging methods andsystems allow for ultrasound signals to be both transmitted and receivedfrom physically and logically separate apertures.

SUMMARY OF THE DISCLOSURE

The various embodiments of systems and methods herein provide theability to perform high resolution three-dimensional ultrasound imagingat frame rates sufficient to capture details of moving objects.Traditional scanline-based ultrasound imaging methods are limited torelatively slow frame rates due to the need to transmit and receive manyscanlines to obtain a single two-dimensional plane. Extending suchtechniques to obtain imaging data from a complete 3D volume results ineven slower frame rates due to the need to image many 2D slices.

As an example, assume that one needs to collect data from a cube oftissue 10 cm on a side at a depth ranging from 5 cm to 15 cm. Ifscanlines are transmitted from a common center, the shape that would beexplored would be a truncated pyramid instead of a shape with comparablethickness in the proximal and distal regions. The tissue may be sampledwith beams that are 2 mm (or less) apart on the distal face of the cube.To cover the distal surface one would need at least 50×50 directed beamsor 2500 directed pulses. With a maximum pulse rate of approximately 2500pulses/sec (which may be constrained by the speed of sound in tissue,the expected signal attenuation, and the background noise level), all ofthe required data may be collected in about one second. This collectiontime may be adequate for non-moving tissue such as bone, liver, etc.,but is not fast enough to capture motion in arteries, or organs such askidneys and especially the heart, or in moving joints or muscles.

On the other hand, with ping-based imaging, a single ping, propagatingsubstantially uniformly in three dimensions, can insonify the entirevolume, and dynamic beamforming (focusing) can identify the sources ofthe echo returns. Using ping-based imaging techniques, a minimum ofthree pings may be needed to obtain data for a 3D volume, while aminimum of two pings may be needed to obtain data for a 2D slice. Inpractical terms, ten to fifty (or more) pings may be used to achieve adesired image quality. For example, the use of 25 pings at a rate of2500 pings per second may require only 0.01 seconds to acquire all thedata for the entire 10 cm cube of tissue. For this particular example,data collection may be 100 times faster than with the scanline-basedmethod.

Using ping-based ultrasound imaging techniques, both 2D and 3D framerates may be increased substantially so as to allow for imaging of 3Dvolumes in real-time. Furthermore, by applying multiple aperture imagingtechniques (e.g., transmitting and receiving ultrasound signals throughmultiple, spatially or physically separated acoustic windows), theresolution of such real-time 3D images may be dramatically improvedrelative to single-aperture techniques.

Ping-based multiple aperture ultrasound imaging can provide verypowerful real-time three-dimensional imaging capabilities as describedabove. The benefits of ping-based multiple aperture ultrasound imagingmay be achieved by using transducer probes with overall dimensions muchlarger than traditional ultrasound probes. For example, ping-basedmultiple aperture ultrasound imaging may be beneficially used withprobes having an active imaging surface in excess of 100 cm².Traditionally, ultrasound elements in a probe are spaced as closetogether as possible, typically significantly less than (and generallyno more than) half a wavelength of the ultrasound frequency being used.

However, using traditional element-to-element spacing in such a largeprobe would require a cable far too thick for the cable to be usable.Although some tricks may be used to reduce the number of individualconductors required in a cable, a better solution is to increase theallowed spacing between elements, thereby reducing the total number ofelements in an array. Use of sparse arrays with traditionalscanline-based imaging methods suffers from substantial complications,artifacts, and low resolution and is therefore not generally practical.Based on the research into the use of sparse arrays scanline-basedphased array techniques, one would expect the use of sparse arrays withping-based multiple aperture ultrasound imaging techniques to suffersimilar difficulties, but that is unexpectedly not the case. In fact,sparse arrays can be used quite effectively with ping-based multipleaperture ultrasound imaging techniques as described herein.

In some embodiments, sparse arrays of transducer elements may bebeneficial in providing an ultrasound probe with a wide total aperturewhile containing a manageable number of transducer elements.

The following disclosure provides various embodiments of ultrasoundprobe configurations, methods of making such probes, and methods ofusing such probes to perform high-frame-rate, high-resolution, real-time2D, 3D and 4D ultrasound imaging.

An ultrasound transducer probe is provided, comprising an array ofultrasound transducing micro-elements, where each micro-element has adiameter less than 500 microns, a first group of micro-elementselectrically connected to a first signal conductor, a second group ofmicro-elements electrically connected to a second signal conductor, thesecond signal conductor being electrically separate from the firstsignal conductor, and a third group of micro-elements positioned betweenthe first group and the second group, the third group of micro-elementsbeing permanently disconnected from any signal conductors.

In some embodiments, each micro-element has a diameter between 25microns and 200 microns.

In other embodiments, some of the micro-elements of the first group aredifferently sized than other micro-elements of the first group, whereinthe size of a micro-element corresponds its fundamental operatingfrequency.

In one embodiment, the micro-elements of the first group are connectedto a first ground conductor and the micro-elements of the second groupare connected to a second ground conductor not electrically connected tothe first ground conductor. In another embodiment, the first group ofmicro-elements includes more micro-elements than the second group. Insome embodiments, the first group of micro-elements collectively forms adedicated transmit element and the second group of micro-elementscollectively forms a dedicated receive element.

In one embodiment, the probe comprises a fourth group of micro-elementselectrically connected to the first signal conductor by a switch that,when closed causes the fourth group to form a combined element withfirst group. In some embodiments, the micro-elements of the fourth groupcollectively surround the micro-elements of the first group. In anotherembodiment, the fourth group of micro-elements is adjacent to the firstgroup of micro-elements. In one embodiment, the fourth group ofmicro-elements is contiguous with the first group of micro-elements.

In some embodiments, the combined element has a different shape than thefirst group alone. In other embodiments, the combined element has ashape that is the same as a shape of the first group but a differentsize.

An ultrasound imaging system is also provided, comprising a transducerprobe having a first array segment and a second array segment separatedfrom the first array segment by a gap of open space, the first andsecond array segments secured to at least one structural housing memberrigidly holding the first and second arrays in fixed positions relativeto one another, and an imaging control system containing instructionsto: transmit an unfocused ultrasound ping from a transmit apertureapproximating a point-source into an object to be imaged, receivingechoes of the ping from reflectors directly below gap with receivetransducer elements on both the first array segment and the second arraysegment, producing a volumetric image of the region below the gap bycombining echo data from echoes received by receive elements on botharray segments.

In one embodiment, each array segment comprises an array ofmicro-elements as in any of the previous embodiments.

An ultrasound imaging probe is also provided, comprising a sparse arrayof ultrasound transducer elements in which less than 50% of potentialelement positions are occupied by active transducer elements, the sparsearray having a first plurality of elements designated as transmitelements and a second plurality of elements designated as receiveelements, and wherein no more than N of the receive elements areequidistant to any one transmit element, wherein N is an integer between1 and 100.

In one embodiment, N is 1, 2, 3, 4, or 5.

In another embodiment, spacing between adjacent elements arepseudo-random distances. In other embodiments, spacing between adjacentelements are non-repeating distances based on a non-repeating numbersequence.

In one embodiment, the transmit elements and the receive elements aremade of bulk piezoelectric material.

In other embodiments, each transmit element and each receive element ismade up of a plurality of micro-elements.

In one embodiment, at least one transmit element or at least one receiveelement is made up of two sub-groups of micro-elements.

In other embodiments, at least one transmit element or at least onereceive element comprises a first plurality micro-elements operating ata first frequency and a second plurality of microelements operating at asecond frequency different than the first frequency.

In additional embodiments, at least two of the designated transmitelements are configured to transmit different frequencies of ultrasoundthan a remainder of the transmit elements.

In other embodiments, spacing between adjacent elements is irregular.

Another ultrasound imaging probe is provided, comprising a sparse arrayof ultrasound transducer elements in which adjacent transducer elementsare spaced by a distance of greater than half a maximum wavelength atwhich any element of the array is configured to operate, the sparsearray having a first plurality of elements designated as transmitelements and a second plurality of elements designated as receiveelements.

In one embodiment, spacing between adjacent elements are pseudo-randomdistances. In other embodiments, spacing between adjacent elements arenon-repeating distances based on a non-repeating number sequence.

In one embodiment, the transmit elements and the receive elements aremade of bulk piezoelectric material.

In another embodiment, each transmit element and each receive element ismade up of a plurality of micro-elements.

In some embodiments, at least one transmit element or at least onereceive element is made up of two sub-groups of micro-elements. Inanother embodiment, at least one transmit element or at least onereceive element comprises a first plurality micro-elements operating ata first frequency and a second plurality of microelements operating at asecond frequency different than the first frequency.

In additional embodiments, at least two of the designated transmitelements are configured to transmit different frequencies of ultrasoundthan a remainder of the transmit elements.

In other embodiments, spacing between adjacent elements is irregular.

An ultrasound imaging method is also provided, comprising transmitting afirst ultrasound ping from a transmit aperture approximating a pointsource at a first time, receiving echoes of the first ultrasound pingwith a first transducer element between the first time and a secondtime, the first transducer element coupled to a first receive channel bya first signal conductor, closing a switch at the second time to connecta second transducer element to the first signal conductor, the secondtransducer element surrounding the first transducer element, receivingechoes of the first ultrasound ping with the first transducer elementand the second transducer element between the second time and a thirdtime, and producing an image from the echoes received between the firsttime and the third time and displaying the image.

In some embodiments, first transducer element has a circular shape, andthe second transducer element has a ring shape concentric with the firsttransducer element.

In an additional embodiment, the first transducer element comprises afirst group of micro-elements electrically connected to the first signalconductor. In some embodiments, the second transducer element comprisesa second group of micro-elements electrically connectable to the firstsignal conductor via the switch.

In another embodiment, echoes received between the first time and thesecond time are near-field echoes.

In some embodiments, the image is a volumetric image. In anotherembodiment, the image is a two-dimensional image.

An ultrasound imaging method is provided, comprising transmitting afirst ultrasound ping from a transmit aperture approximating a pointsource at a first time, receiving and storing echoes of the firstultrasound ping with a first transducer element, the first transducerelement coupled to a first receive channel by a first signal conductor,receiving and storing echoes of the first ultrasound ping with a secondtransducer element that surrounds the first transducer element, thesecond transducer element coupled to a second receive channel by asecond signal conductor, retrieving first echoes received by the firstelement between a first time and a third time, retrieving second echoesreceived by the second element between a second time and the third time,the second time occurring between the first time and the third time,combining the first echoes received between the second time and thethird time with the second echoes, and producing an image from thecombined echoes and displaying the image.

In some embodiments, the first transducer element has a circular shape,and the second transducer element has a ring shape concentric with thefirst transducer element.

In other embodiments, the first transducer element comprises a firstgroup of micro-elements electrically connected to the first signalconductor.

In one embodiment, the second transducer element comprises a secondgroup of micro-elements electrically connectable to the first signalconductor via the switch.

In another embodiment, echoes received between the first time and thesecond time are near-field echoes.

In some embodiments, the image is a volumetric image. In anotherembodiment, image is a two-dimensional image.

An ultrasound imaging method is also provided, comprising transmitting afirst ultrasound ping from a transmit aperture approximating a pointsource at a first time, receiving echoes of the first ultrasound pingwith a first transducer element between the first time and a secondtime, the first transducer element coupled to a first receive channel bya first signal conductor, opening a switch between the first transducerelement and the first signal conductor and simultaneously closing aswitch between a second transducer element and the signal conductor, thesecond transducer element being larger than the first transducerelement, receiving echoes of the first ultrasound ping with the secondtransducer element between the second time and a third time, andproducing an image from the echoes received by both the first transducerelement and the second transducer element between the first time and thethird time and displaying the image.

In one embodiment, opening a switch between the first transducer elementand the first signal conductor and simultaneously closing a switchbetween the second transducer element and the signal conductor comprisesoperating a single switch.

In one embodiment, the image is a volumetric image. In anotherembodiment, the image is a two-dimensional image.

An ultrasound imaging method is also provided, comprising transmitting afirst ultrasound ping from a transmit aperture approximating a pointsource at a first time, receiving and storing echoes of the firstultrasound ping with a first transducer element, the first transducerelement coupled to a first receive channel by a first signal conductor,receiving and storing echoes of the first ultrasound ping with a secondtransducer element that is larger than the first transducer element, thesecond transducer element coupled to a second receive channel by asecond signal conductor, retrieving first echoes received by the firsttransducer element between a first time and a second time, retrievingsecond echoes received by the second transducer element between thesecond time and a third time, the second time occurring between thefirst time and the third time, producing an image from the first echoesand the second echoes, and displaying the image.

In one embodiment, the image is a volumetric image. In anotherembodiment, the image is a two-dimensional image.

An ultrasound imaging method is provided, comprising transmitting anunfocused ultrasound ping from a transmitter approximating a pointsource into an object to be imaged, receiving echoes of the ping at afirst receive element and a second receive element, where a line betweenthe first receive element and the second receive element defines anaxis, retrieving position data defining a position of the first receiveelement and the second receive element relative to a common coordinatesystem, identifying a first echo sample corresponding to a firstreflector received at the first element, and identifying a second echosample corresponding to the same first reflector received at the secondelement, determining a first time-of-arrival at which the first sampleecho was received at the first receive element, determining secondtime-of-arrival at which the second sample echo was received at thesecond receive element, comparing the first time-of-arrival and thesecond time-of-arrival to determine which of the echo samplescorresponding to the first reflector was received first, determiningthat the first element is closest to the first reflector based on thecomparison of times-of-arrival, assigning a greater weight to the firstecho sample than the second echo sample, and producing an image of thereflector from the weighted first echo sample and the weighted secondecho sample, and displaying the image.

In some embodiments, the method further comprises assigning a greaterweight to the second echo sample than a third echo sample received by athird element that is further from the first element than from thesecond element along the axis, and producing the image of the reflectorfrom the weighted first echo sample, the weighted second echo sample,and the weighted third echo sample.

In one embodiment, the first time-of-arrival is based on explicit timinginformation in stored echo data.

In another embodiment, the first time-of-arrival is based on implicittiming information in stored echo data.

An ultrasound imaging system is also provided, comprising an array ofultrasound transducer elements, a first transducer element in the arrayhaving a long axis and a short axis, wherein the first transducerelement produces a first phase signature in response to ultrasoundsignals received from a first region of an imaged object and a secondphase signature in response to ultrasound signals received from a secondregion of the imaged object, an imaging system configured to transmitultrasound signals into the object from at least one transmit apertureapproximating a point-source and to receive signals produced by thefirst transducer element in response to echoes of signals transmittedfrom the at least one transmit aperture, wherein the imaging system isfurther configured to determine whether a given reflector is located inthe first region or the second region based on a phase signatureproduced by the first transducer element in response to an echo of thereflector received by the first transducer element, and wherein theimaging system is further configured to apply weights to echoes of thereflector received by other receive elements in the array based on thedetermination of the reflector being located in the first region or thesecond region, and to produce an image based on the weighted echoes.

In some embodiments, the first region and the second region arequadrants, and wherein the first transducer element further produces athird phase signature in response to ultrasound signals received from athird quadrant of the imaged object, and a fourth phase signature inresponse to ultrasound signals received from a fourth quadrant of theimaged object, and wherein the first and second quandrants correspond toregions of the object adjacent opposite ends of the short axis and thethird and fourth quadrants correspond to regions of the object adjacentopposite ends of the long axis.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of the invention are set forth with particularity inthe claims that follow. A better understanding of the features andadvantages of the present invention will be obtained by reference to thefollowing detailed description that sets forth illustrative embodiments,in which the principles of the invention are utilized, and theaccompanying drawings of which:

FIG. 1 is a schematic diagram illustrating a sparse array of ultrasoundtransducer elements made up of micro-elements.

FIG. 2 is a schematic diagram illustrating a sparse array of ultrasoundtransducer elements made up of micro-elements with no micro-elementsbetween identified elements.

FIG. 3 is a schematic diagram illustrating rectangular a sparse arrayarrangement of ultrasound transducer elements represented as squares.

FIG. 4 is a schematic diagram illustrating a rectangular sparse arrayarrangement of ultrasound transducer elements.

FIG. 5 is a schematic diagram illustrating an elliptical sparse arrayarrangement of ultrasound transducer elements.

FIG. 6 is a schematic diagram illustrating a circular sparse arrayarrangement of ultrasound transducer elements.

FIG. 7 is a schematic diagram illustrating a concave or convexthree-dimensional surface sparse array arrangement of ultrasoundtransducer elements.

FIG. 8 is a plan view illustration of a sparse array of regularly-spacedtransmit and receive transducer elements.

FIG. 9 is a schematic diagram illustrating an example multi-frequencytransmit transducer element made up of a plurality of micro-elements ofdifferent sizes.

FIG. 10A is a schematic illustration of an embodiment of an electriccircuit that may be used to connect a group of concentric receiveelements to separate receive channels of a receive subsystem.

FIG. 10B is a schematic illustration of an embodiment of an electriccircuit that may be used to connect a group of concentric receiveelements to a receive channel of a receive subsystem.

FIG. 11A is a schematic illustration of an embodiment of an electriccircuit that may be used to connect a group of circular receive elementsof different sizes to separate receive channels of a receive subsystem.

FIG. 11B is a schematic illustration of an embodiment of an electriccircuit that may be used to connect a group of circular receive elementsof different sizes to a single receive channel of a receive subsystem.

FIG. 12 is a schematic diagram illustrating a plurality of receivetransducer elements in a grid pattern, showing axes for determiningestimated reflector locations.

FIG. 13 is a schematic diagram illustrating an asymmetrical receivetransducer element with a long axis and a short axis.

FIG. 14 is a schematic diagram illustrating a constellationconfiguration of ultrasound transmit elements surrounded by receiveelements grouped into overlapping receive apertures based, at least inpart, on proximity to transmit elements.

FIG. 15 is a schematic illustration of a circular ultrasound imagingprobe having a central opening.

FIG. 16A is a schematic illustration showing a top plan view of anultrasound imaging probe having two probe segments separated by a gapand joined to a bridge handle.

FIG. 16B is a schematic illustration showing a bottom plan view of anultrasound imaging probe having two probe segments separated by a gapand joined to a bridge handle.

FIG. 17 is a schematic block diagram illustrating example components inan embodiment of a multiple aperture imaging system.

DETAILED DESCRIPTION

The various embodiments will be described in detail with reference tothe accompanying drawings. References made to particular examples andimplementations are for illustrative purposes, and are not intended tolimit the scope of the invention or the claims.

The present disclosure provides systems and methods for improving thequality of real-time two-dimensional and three-dimensional ultrasoundimaging through the use of sparse arrays of various construction,including arrays in which each “element” is made up of a plurality ofmicro-elements arranged to be operated in concert with one another.

Although the various embodiments are described herein with reference toultrasound imaging of various anatomic structures, it will be understoodthat many of the methods and devices shown and described herein may alsobe used in other applications, such as imaging and evaluatingnon-anatomic structures and objects. For example, the variousembodiments herein may be applied to non-destructive testingapplications such as evaluating the quality, integrity, dimensions, orother characteristics of various structures such as welds, pressurevessels, pipes, structural members, beams, etc. The systems and methodsmay also be used for imaging and/or testing a range of materialsincluding human or animal tissues, solid metals such as iron, steel,aluminum, or titanium, various alloys or composite materials, etc.

Introduction to Key Terms

The following paragraphs provide useful definitions for some terms usedfrequently herein. Other terms may also be defined as they are used.

As used herein the terms “ultrasound transducer” and “transducer” maycarry their ordinary meanings as understood by those skilled in the artof ultrasound imaging technologies, and may refer without limitation toany single component capable of converting an electrical signal into anultrasonic signal and/or vice versa. For example, in some embodiments,an ultrasound transducer may comprise a piezoelectric device. In otherembodiments, ultrasound transducers may comprise capacitivemicro-machined ultrasound transducers (CMUT), other micro-machinedtransducers made of electroactive materials such as piezoelectricmaterials, ferroic materials, ferroelectric materials, pyroelectricmaterials, electrostrictive materials, or any other transducing materialor device capable of converting ultrasound waves to and from electricalsignals.

Transducers are often configured in arrays of multiple individualtransducer elements. As used herein, the terms “transducer array” or“array” generally refers to a collection of transducer elements attachedto a common support structure. An array may typically (though notnecessarily) comprise a plurality of transducer elements mounted to acommon backing plate or substrate. Such arrays may have one dimension(1D), two dimensions (2D), 1.X dimensions (1.XD) or three dimensions(3D) as those terms are used elsewhere herein and/or as they arecommonly understood in the art. Other dimensioned arrays as understoodby those skilled in the art may also be used. Annular arrays, such asconcentric circular arrays and elliptical arrays may also be used. Insome cases, transducer arrays may include irregularly-spaced transducerelements, sparsely positioned transducer elements (also referred to assparse arrays), randomly spaced transducer elements, or any othergeometric or random arrangement of transducer elements. Elements of anarray need not be contiguous and may be separated by non-transducingmaterial or empty space.

An element of a transducer array may be the smallest discretelyfunctional component of an array. For example, in the case of an arrayof piezoelectric transducer elements, each element may be a singlepiezoelectric crystal or a single machined section of a piezoelectriccrystal. As another example, in an array made up of a plurality ofmicro-elements (e.g., micro-machined elements, micro-dome elements, orother micro-sized elements), a group of micro-elements may beelectrically coupled so as to operate collectively as a singlefunctional element. In such a case, a group of collectively-operatingmicro-elements may be referred to as a single “element.”

As used herein, the terms “transmit element” and “receive element” maycarry their ordinary meanings as understood by those skilled in the artof ultrasound imaging technologies. The term “transmit element” mayrefer without limitation to an ultrasound transducer element which atleast momentarily performs a transmit function in which an electricalsignal is converted into an ultrasound signal. Similarly, the term“receive element” may refer without limitation to an ultrasoundtransducer element which at least momentarily performs a receivefunction in which an ultrasound signal impinging on the element isconverted into an electrical signal.

In cases where imaging is performed by transmitting “ping-based” or“point-source transmission” ultrasound signals, the term “transmitelement” may refer to a single element or to a plurality of elementsoperated together to form the desired waveform transmission. Forexample, a plurality of transducer elements may be activatedsimultaneously or with delays selected to produce a circular orspherical waveform in the region of interest. Such a plurality oftransducers, when operated together to form such a waveform, may have acollective acoustic center which is the apparent point-source origin ofthe transmitted waveform. Phrased differently, one or more transmitelements may approximate a point-source transmitter if an unfocusedspherical waveform produced by the one or more transmit elements appearsto have originated from a point source.

Transmitted ultrasound signals may be focused in a particular direction,or may be unfocused, transmitting in all directions or a wide range ofdirections. Transmission of ultrasound into a medium may also bereferred to herein as “insonifying.” An object or structure whichreflects ultrasound waves may be referred to as a “reflector” or a“scatterer.”

As used herein, terms referring to a “position” or “location” of atransducer element refer to an acoustic center position exhibited by theelement. In some cases, an acoustic center position of an element may beprecisely coincident with a mechanical or geometric center of theelement, group of elements, or group of micro-elements. However, in manycases an acoustic center position of an element may be different than amechanical or geometric center of the element due to various factorssuch as manufacturing irregularities, damage, irregular elementgeometries, or other factors. Acoustic center positions of elements maybe determined using various calibration techniques such as thosedescribed in US Patent Application Publication 2014/0043933, titled“Calibration of Multiple Aperture Ultrasound Probes,” U.S. Pat. No.9,282,945, titled “Calibration of Ultrasound Probes,” or other methods.

As used herein, the term “aperture” may refer to a single transducerelement or a group of transducer elements that are collectively managedas a common group by imaging control electronics. For example, in someembodiments an aperture may be a grouping of elements which may bephysically separate and distinct from elements of an adjacent aperture.However, adjacent apertures need not necessarily be physically separateor distinct. Conversely, a single aperture may include elements of twoor more physically separate or distinct transducer arrays or elementsspaced from one another by any distance or different distances. In somecases, two or more elements need not be adjacent to one another to beincluded in a common aperture with one another. For example, distinctgroups of transducer elements (e.g., a “left aperture”) may beconstructed from a left array, plus the left half of a physicallydistinct center array, while a “right aperture” may be constructed froma right array, plus the right half of a physically distinct centerarray.

As used herein, the terms “receive aperture,” “insonifying aperture,”and/or “transmit aperture” are used herein to mean an individualelement, a group of elements within an array, or even entire arrays,that perform the desired transmit or receive function as a group. Insome embodiments, such transmit and receive apertures may be created asphysically separate components with dedicated functionality. In otherembodiments, any number of transmit and/or receive apertures may bedynamically defined electronically as needed. In other embodiments, amultiple aperture ultrasound imaging system may use a combination ofdedicated-function and dynamic-function apertures. In some cases,elements may be assigned to different apertures during two or more pingcycles (as defined below).

As used herein, the term “ping cycle” refers to a cycle that begins withthe transmission of a ping from a transmit aperture approximating apoint source and ends when all available (or all desired) echoes of thattransmitted ping have been received by receive transducer elements. Inmany cases, ping cycles may be distinct and separated by some timeperiod. In other cases, ping cycles may overlap one another in time.That is, an N+1th ping cycle may begin (with transmission of a ping)before an Nth ping cycle is completed (i.e., before all echoes of theNth ping are received).

In various embodiments, a single “image” or “image frame” may beproduced from the echoes of one or more transmitted pings. Therefore, an“imaging cycle” or “image cycle” may refer to a cycle that begins withthe transmission of a first ping that will contribute to an image andmay end with the reception of echoes of a final ping contributing to thesame image. In various embodiments, a single imaging cycle may includeone, two, three, four, five, or more ping cycles.

As used herein, the term “total aperture” refers to the overall size ofall imaging apertures in a probe. In other words, the term “totalaperture” may refer to one or more dimensions defined by a maximumdistance between the furthest-most transducer elements of anycombination of transmit and/or receive elements used for a particularimaging cycle. Thus, the total aperture may be made up of any number ofsub-apertures designated as send or receive apertures for a particularcycle. In the case of a single-aperture imaging arrangement, the totalaperture, sub-aperture, transmit aperture, and receive aperture may allhave the same dimensions. In the case of a multiple aperture imagingarrangement, the dimensions of the total aperture may include the sum ofthe dimensions of all send and receive apertures plus any space betweenapertures.

In some embodiments, two apertures may be located adjacent to oneanother on a continuous array. In other embodiments, two apertures mayoverlap one another on a continuous array, such that at least oneelement functions as part of two separate apertures. The location,function, number of elements and physical size of an aperture may bedefined dynamically in any manner needed for a particular application.

Elements and arrays described herein may also be multi-function. Thatis, the designation of transducer elements or arrays as transmitters inone instance does not preclude their immediate re-designation asreceivers in the next instance. Moreover, embodiments of control systemsherein include the capabilities for making such designationselectronically based on user inputs, pre-set scan or resolutioncriteria, or other automatically determined criteria.

As used herein, the “image-able field” of the imaging system may be anyarea or volume of an imaged object or substance that may practically beimaged by the imaging system. For a ping-based imaging system asdescribed herein, the term “image-able field” may be synonymous with theterm “insonified region.” The term “region of interest” may refer to atwo-dimensional or three-dimensional region within the image-able field.The extents of an image-able field relative to a probe may be imposed byphysical limits (e.g., based on signal-to-noise ratios or attenuationrates) or may be chosen logical limits (e.g., based on a desired regionof interest).

As used herein, the term “pixel” refers to a region of two-dimensionalspace within an image-able field of the imaging system. The term “pixel”is not intended to be limited to a pixel of a display device, and mayrepresent a region of a real-world-scale object that is either larger orsmaller than a display pixel. A “pixel” may represent a region of theimage-able field of any real-world size, and in some cases may representa region smaller than any resolvable object of the imaging system.Pixels may be, but need not necessarily be square or rectangular, andmay have any shape allowing for contiguous two-dimensionalrepresentation of the image-able field. In some cases, data representinga pixel may not be displayed, but may still be processed as a unit andreferred to as a “pixel.”

As used herein, the term “voxel” refers to a region of three-dimensionalspace within an image-able field of the imaging system. The term “voxel”is not intended to be limited to any particular portion of atwo-dimensional or three-dimensional display device, and may represent aregion of a real-world-scale object that is either larger or smallerthan a display voxel. A “voxel” may represent a three-dimensional regionof the image-able field of any real-world size, and in some cases mayrepresent a region smaller than any resolvable object of the imagingsystem. Voxels may be, but need not necessarily be three-dimensionalsquare or rectangular prisms. Voxels may have any shape allowing forcontiguous three-dimensional representation of the image-able field. Insome cases, data representing a voxel may not be displayed, but maystill be processed as a unit and referred to as a “voxel.”

As used herein, the terms “pixel location” and “voxel location” (or“position”) refer to a location within the image-able field that isidentifiable by a coordinate system, which may be a Cartesian coordinatesystem or any other coordinate system. Unless otherwise specified,references to a location of a pixel or voxel may generally refer to acenter-point (e.g., center-of-mass, circular center, etc.) of the pixelor voxel.

As used herein, a pixel may be described as “intersecting” a voxel. Atwo-dimensional pixel may be defined as intersecting a three-dimensionalvoxel using any desired convention. For example, for square pixels andcubic voxels, a pixel intersecting a voxel may be a square face of thevoxel or any other square or rectangle passing through the voxel. If acoordinate system used for pixels is different than a coordinate systemused for voxels, then one pixel may intersect multiple voxels.

As used herein, the term “echo” refers to an ultrasound wavefront or ananalog or digital representation of an ultrasound wavefront that arrivesat a receive transducer element. Because imaging methods describedherein allow for an extremely wide range of probe configurations, someultrasound signals arriving at a receive transducer element mayoriginate at a transmit transducer element on an opposite side of animaged object. Such wavefronts are also intended to be included withinthe definition of an “echo” even if such wavefronts may also bedescribed as “transmitted” or “deflected.”

As used herein, the terms “reflector” and “scatterer” refer to aphysical portion of a physical object being imaged. When struck by awavefront, reflectors and scatterers will tend to re-radiate a wavefrontin a direction generally dictated by physics. The terms are not intendedto limit the relative geometry or positions of transmitters, scatterers,and reflectors.

As used herein, the verb terms “reflect” and “scatter” refer to theeffect of a scatterer on a propagating ultrasound wavefront. In somecases, a wavefront that is only slightly deflected (e.g., forming acombined transmit element/scatterer/receive element angle approaching180°) by a scatterer may still be described as having been “reflected”by that scatterer (or “reflector”).

As used herein, the term “sample” refers to a digital data element in aphysical volatile or non-volatile storage medium. Unless contextsuggests otherwise, “samples” described herein generally refer to dataelements representing a discrete portion of a received ultrasoundwaveform. A time-varying electrical signal produced by a transducerelement vibrating in response to a received ultrasound wavefront may bequantified and digitally sampled at a sample rate in order to produce aseries of digital values representing the received time-varyingelectrical signal. Those values may be referred to as “samples.” In somecases, a “sample” may include an interpolated value in between twodigitally stored sample values.

If digital sampling is done at a known sample rate (usually, but notnecessarily a consistent sample rate), the position of each sample(e.g., as measured by a location in memory device, or a position in asequence of values) may be directly related to an arrival time of thewavefront segment responsible for each sample value.

As used herein, the term “beamform” refers to the process of determininga value for pixels or voxels based on a sample value (directly stored orinterpolated), the known acoustic center position of a transmit elementresponsible for the sample value, and the known acoustic center positionof a receive element responsible for the sample value. Beamforming isdescribed in further detail elsewhere herein.

As used herein, the term “image” (as a noun) refers to a human-visiblegraphical representation of a physical object or a series ofnon-transitory digital values stored on a physical storage medium thatmay be interpreted by software and/or an image processor to produce sucha graphical representation. As used herein, the term “image” does notnecessarily imply any particular degree of quality or human-readability.An “image” may refer to a two-dimensional representation (e.g., across-section, in some cases) or a three-dimensional volumetricrepresentation of an object. Therefore, a “volumetric image” may includea visible representation of a three-dimensional point cloud or digitaldata representing the three-dimensional point cloud. As used herein, theterms “image” and “imaging” (in verb form) refer to a process thatresults in an image.

Introduction to Point-Source Transmission Ultrasound Imaging

In various embodiments, point-source transmission ultrasound imaging,otherwise referred to as ping-based ultrasound imaging, provides severaladvantages over traditional scanline-based imaging. Point-sourcetransmission differs in its spatial characteristics from a “phased arraytransmission” which focuses energy in a particular direction from thetransducer element array along a directed scanline. A point-source pulse(also referred to herein as a “ping”) may be transmitted so as togenerate either a two-dimensional circular wavefront or athree-dimensional spherical wavefront, thereby insonifying as wide anarea as possible in the two-dimensional or three-dimensional region ofinterest. Echoes from scatterers in the region of interest may return toall of the elements of receive apertures (or all of those elements notblocked by obstacles preventing transmission of the echoes). Thosereceived echo signals may be filtered, amplified, digitized and storedin short term or long term memory (depending on the needs orcapabilities of a particular system).

Images may then be reconstructed from received echoes by determiningpositions of reflectors responsible for received echo samples. Theposition of each reflector responsible for a digital echo sample may becalculated based on the arrival time of the received echo sample (whichmay be inferred based on a sample position and a known sampling rate)relative to the known time at which the ping was transmitted, theacoustic position of the transmit element(s) responsible for the echosample, and the acoustic position of the receive element responsible forthe echo sample. This process of determining positions of reflectors isgenerally referred to herein as beamforming.

Beamforming may be performed by a software-based, firmware-based, orhardware-based dynamic beamforming technique, in which a beamformer'sfocus may be continuously changed to focus at a particular pixelposition corresponding to a reflector position. Such a beamformer may beused to plot the position of echoes received from point-source pings. Insome embodiments, such a dynamic beamformer may plot the locus of eachecho signal based on a round-trip travel time of the signal from thetransmitter to an individual receive transducer element.

In the two-dimensional imaging case, for a given echo sample produced bya transmit transducer element and a receive transducer element, thelocus of possible positions of the target reflector responsible for theecho sample will be an ellipse mathematically defined by two foci. Afirst focus of the ellipse will be at the position of the transmittransducer element and the second focus will be at the position of thereceive transducer element. Although several other possible reflectorpositions lie along the same ellipse, echoes of the same targetreflector will also be received by other receive transducer elements.The slightly different positions of each receive transducer elementmeans that each receive element will define a slightly different ellipsefor the target reflector. Accumulating the results by summing theellipses for multiple receive elements at slightly different positionswill indicate an intersection of the ellipses for a reflector. As echosamples from more receive elements are combined with the first, theintersecting ellipses will converge towards a point at which the targetreflector is located. Similarly, echoes of pings transmitted fromdifferent transmit element positions may also be combined to furtherrefine reflector points. The target reflector position may be correlatedwith a pixel location representing the reflector. The combined samplevalues may be used to determine a display intensity for a display pixelat the pixel location. The echo amplitudes received by any number oftransmit positions and receive elements may thereby be combined to formeach pixel. In other embodiments the computation can be organizeddifferently to arrive at substantially the same result.

Various algorithms may be used for combining echo signals received byseparate receive elements. For example, some embodiments may processecho-signals individually, plotting each echo signal at all possiblelocations along its ellipse, then proceeding to the next echo signal.Alternatively, each pixel location may be processed individually,identifying and processing all echo samples potentially contributing tothat pixel location before proceeding to the next pixel location.

Image quality may be further improved by combining images formed by thebeamformer from one or more subsequent transmitted pings, transmittedfrom the same or a different point-source (or multiple differentpoint-sources). Still further improvements to image quality may beobtained by combining images formed by more than one receive aperture.

An important consideration is whether the summation of images fromdifferent pings, different transmit point-sources or different receiveapertures should be coherent summation (phase sensitive) or incoherentsummation (summing magnitude of the signals without phase information).

The decision as to whether to use coherent or incoherent summation maybe influenced by the lateral extent/size of the receive aperture(s)and/or the transmit aperture(s). In some embodiments, it may beconvenient to confine the size of an aperture to conform to theassumption that the average speed of sound is substantially the same forevery path from a scatterer to each element of the transmit or receiveaperture. For narrow receive apertures this simplifying assumption iseasily met. However, as the width of the receive aperture increases, aninflection point may be reached (referred to herein as the “maximumcoherent aperture width” or “maximum coherence width”), beyond whichpaths traveled by echoes of a common reflector will necessarily passthough different types of tissue having intrinsically different speedsof sound when returning to the elements furthest apart from one another.When this difference results in receive wavefront phase shiftsapproaching or exceeding 180 degrees, additional receive elementsextended beyond the maximum coherence width will actually degrade theimage rather than improve it.

The same maximum coherent aperture size considerations may also apply tothe size of transmit apertures, which may include a plurality oftransducer elements. In the case of two-dimensional transducer arraysused in three-dimensional imaging (or 3D data collection), it may beuseful to define a maximum coherent aperture size in two dimensions.Thus, in various embodiments a maximum coherent aperture may be definedas a group of transducer elements in a square, circle, polygon or othertwo-dimensional shape with a maximum distance between any two elementssuch that phase cancellation will be avoided when echo data received atthe elements of the aperture are coherently combined.

Therefore, in order to realize the benefits (e.g., in terms of increasedspatial resolution) of a wide probe with a total aperture width fargreater than the maximum coherent aperture width, the full probe widthmay be physically or logically divided into multiple transmit and/orreceive apertures, each of which may be limited to an effective widthless than or equal to the maximum coherent aperture width, and thussmall enough to avoid phase cancellation of received signals.

The maximum coherence width can be different for different patients (ordifferent test objects), for different probe positions on the samepatient, and for other variables such as ultrasound frequency. In someembodiments, a compromise width may be determined for a given probeand/or imaging system. In other embodiments, a multiple apertureultrasound imaging control system may be configured with a dynamicalgorithm to subdivide the available elements into groups that are smallenough to avoid significant image-degrading phase cancellation. Invarious embodiments, a particular coherent aperture size may bedetermined automatically by a control system, or manually through userinput via a user control such as a dial or slider.

In some embodiments, coherent (phase sensitive) summation may be used tocombine echo data received by transducer elements located on a commonreceive aperture resulting from one or more pings. In some embodiments,incoherent summation may be used to combine echo data or image datareceived by separate receive apertures if such receive apertures aresized and positioned so as to form a combined total aperture that isgreater than a maximum coherence width for a given imaging target.

Two-dimensional ping-based beamforming may implicitly assume that thewavefronts emitted from the point-source are physically circular in theregion of interest. In actuality, the wavefront may also have somepenetration in the dimension orthogonal to the scanning plane (i.e.,some energy may essentially “leak” into the dimension perpendicular tothe desired two-dimensional image plane, which may have the effect ofreducing the effective imaging depth). Additionally, the “circular”wavefront may actually be limited to a semicircle or a fraction of acircle less than 180 degrees ahead of the front face of the transduceraccording to the unique off-axis properties of the transducing materialused. Similarly, a three-dimensional “spherical” wavefront may have anactual shape of a hemisphere or less than a hemisphere within the mediumto be imaged.

Ping-Based Imaging for 3D Ultrasound Imaging

The above description of point-source ultrasound imaging (also referredto herein as “ping-based” imaging) predominantly describestwo-dimensional imaging in which ultrasound signals are focused into anarrow field approximating a plane in a region of an image. Suchtwo-dimensional focusing may be accomplished with lensing or othertechniques. Ping-based imaging can also be extended to real-timethree-dimensional imaging without adding significant complexity.Three-dimensional ping-based imaging can be performed using anultrasound probe with transducer elements spaced from one another in twodimensions. Some example probe configurations are described elsewhereherein.

When a three-dimensional pulse is initiated from a point-source transmittransducer, the resulting semi-spherical wavefront travels into theinsonified region containing a region of interest (ROI) where some ofthe ultrasound energy may be reflected (or deflected) by scatterers inthe ROI. Some of the echoes from the scatterers may travel towardsreceive transducer elements of the probe, where the echoes may bedetected, amplified, digitized, and stored in a short-term or long-termmemory device. Each digitized sample value may represent a scattererfrom the ROI. As in the 2D case, the magnitude of each received sample,along with its time of arrival and the exact positions of the transmitand receive transducers used, may be analyzed to define a locus ofpoints identifying potential positions of the scatterer. In the 3D case,such a locus is an ellipsoid having as its foci the positions of thetransmitter point source and receive transducer element. Each uniquecombination of transmit and receive transducer elements may define aseparate view of the same reflector. Thus, by combining information fromellipsoids produced from multiple transmit-receive transducer elementcombinations, the actual location of each reflector may be moreaccurately represented.

For example, in some embodiments an image in a 3D array of voxels may beassembled in computer memory by beginning with an evaluation of aselected digital sample. The selected digitized sample value may bewritten into every voxel indicated by the corresponding ellipsoiddescribed above. Proceeding to do the same with every other collectedsample value, and then combining all resulting ellipsoids may produce amore refined image. Real scatterers would be indicated by theintersection of many ellipsoids whereas parts of the ellipsoids notreinforced by other ellipsoids would have low levels of signal and maybe treated as noise (i.e., eliminated or reduced by filters, gainadjustments, or other image processing steps).

In other embodiments, the order of computation may be changed bybeginning with a selected voxel in a final 3D image volume to beproduced. For example, for a selected voxel, the closest stored sampleor interpolated sample may be identified for each transmitter/receiverpair. All samples corresponding to the selected voxel may then beevaluated and summed (or averaged) to produce a final representation ofthe voxel. Closeness of a sample to a selected voxel may be determinedby calculating the vector distance from the three-dimensional positionof a transmitter (i.e., the transmitter used to produce the sample) tothe selected voxel position plus the vector distance from the selectedvoxel position to the position of a receiver used to produce the sample.Such a linear distance may be related to the time-divided sample valuesby dividing the total path length by speed of sound through the imagedobject. Using such a method, the samples corresponding to a calculatedtime may be associated with the selected voxel. Once identified, datacorresponding to a common voxel may be combined according to a chosencombining algorithm.

Image Layer Combining

Techniques for determining the location for received echo samples aregenerally referred to herein as beamforming, while techniques forcombining information obtained from multiple transmitter/receivercombinations or from multiple separate pings transmitted using the sametransmitter/receiver combination may generally be referred to as imagelayer combining. In various embodiments, a frame may be made up of anynumber of combined image layers. Frames may be displayed sequentially ata desired frame-rate on a display to form a moving image or video. Theabove-described beamforming processes may beneficially also be used forevaluating pixel values in a 2D cross-sectional slice through a 3Dvolume using raw echo data. In various embodiments, such 2D slices maybe taken at any arbitrary angle or along any curved path through the 3Dvolume. The same techniques may also be used to zoom in (i.e., increasethe size of features) using raw echo data rather than enlargingprocessed pixels or voxels.

Images obtained from different unique combinations of one ping and onereceive element and/or combinations of one ping and one receive aperturemay be referred to herein as “sub-image layers.” Multiple sub-imagelayers may be combined coherently to improve overall image quality(e.g., by combining multiple ellipses or ellipsoids as described above).Additional image layer combining may be performed to further improve thequality of a final image. In the context of image layer combining, theterm “image” may refer to a single two-dimensional pixel, a single voxelof a three-dimensional volume or a collection of any number of pixels orvoxels.

Image layer combining may be described in terms of four image layerlevels. These include base-level image layers, first-level image layers,second-level image layers and third-level image layers. As used herein,the phrase base-level image layer refers to an image obtained bybeamforming echoes received at a single receive element from a singletransmitted ping. As described above, the beamforming process defines anellipse corresponding to each echo sample. Therefore, a base-level imagemay include a series of such ellipses representing all of the echoes ofa single ping received by a single receive element. Such an image maynot be particularly useful for diagnostic imaging purposes, but may beused for other purposes.

A first-level image layer may be formed from echoes received at a singlereceive aperture resulting from a single ping from a single transmitaperture (where a “transmit aperture” can be a composite point-sourcetransmit element, a single-element transmitter, or a group of transmitelements). For a unique combination of a single ping and a singlereceive aperture, the echoes received by all the receive elements in thereceive aperture may be summed coherently to obtain a first-level imagelayer. Alternatively, first-level images may be formed by combining theechoes of two or more pings received at elements of a common receiveaperture.

Multiple first-level image layers resulting from echoes of multipletransmit pings (from the same or different transmit apertures) receivedat a single receive aperture can be summed together to produce asecond-level image layer. Second-level image layers may be furtherprocessed to improve alignment or other image characteristics.

Third-level images may be obtained by combining second-level imagelayers formed with data from multiple receive apertures. In someembodiments, third-level images may be displayed as sequentialtime-domain frames to form a moving image.

In some embodiments, pixels or voxels of a first-level image layer mayalso be formed by summing in-phase and quadrature echo data, that is bysumming each echo with an echo ¼ wavelength delayed for eachreceive-aperture element. In some cases, echo data may be sampled andstored as an in-phase data set and as a separate quadrature data set. Inother cases, if the digital sampling rate is divisible by four, then aquadrature sample corresponding to an in-phase sample may be identifiedby selecting a sample at an appropriate number of samples prior to thein-phase sample. If the desired quadrature sample does not correspond toan existing whole sample, a quadrature sample may be obtained byinterpolation. Combining in-phase and quadrature data for a single image(pixel, voxel or collection of pixels or voxels) may provide theadvantage of increasing the resolution of the echo data withoutintroducing blurring effects. Similarly, samples at values other than ¼wavelength may be combined with in-phase samples in order to improvevarious imaging characteristics.

Combination, summation or averaging of various image layers may beaccomplished either by coherent addition, incoherent addition, or acombination of the two. Coherent addition (incorporating both phase andmagnitude information during image layer summation) tends to maximizelateral resolution, whereas incoherent addition (summing magnitudes onlyand omitting phase information) tends to average out speckle noise andminimize the effects of image layer alignment errors that may be causedby minor variations in the speed of sound through the imaged medium.Speckle noise is reduced through incoherent summing because each imagelayer will tend to develop its own independent speckle pattern, andsumming the patterns incoherently has the effect of averaging out thesespeckle patterns. Alternatively, if the patterns are added coherently,they reinforce each other and only one strong speckle pattern results.

In most embodiments, echoes received by elements of a single receiveaperture are typically combined coherently. In some embodiments, thenumber of receive apertures and/or the size of each receive aperture maybe changed in order to maximize some desired combination of imagequality metrics such as lateral resolution, speed-of-sound variationtolerance, speckle noise reduction, etc. In some embodiments, suchalternative element-to-aperture grouping arrangements may be selectableby a user. In other embodiments, such arrangements may be automaticallyselected or developed by an imaging system.

Variations in the speed of sound may be tolerated by incoherent additionas follows: Summing two pixels coherently with a speed-of-soundvariation resulting in only half a wavelength's delay (e.g.,approximately 0.25 mm for 3 MHz ultrasound) results in destructive phasecancellation, which causes significant image data loss; if the pixelsare added incoherently, the same or even greater delay causes only aninsignificant spatial distortion in the image layer and no loss of imagedata. The incoherent addition of such image layers may result in somesmoothing of the final image (in some embodiments, such smoothing may beadded intentionally to make the image more readable).

At all three image layer levels, coherent addition can lead to maximumlateral resolution of a multiple aperture system if the geometry of theprobe elements is known to a desired degree of precision and theassumption of a constant speed of sound across all paths is valid.Likewise, at all image layer levels, incoherent addition leads to thebest averaging out of speckle noise and tolerance of minor variations inspeed of sound through the imaged medium.

In some embodiments, coherent addition can be used to combine imagelayers resulting from apertures for which phase cancellation is notlikely to be a problem, and incoherent addition can then be used wherephase cancellation would be more likely to present a problem, such aswhen combining images formed from echoes received at different receiveapertures separated by a distance exceeding some threshold.

In some embodiments, all first-level images may be formed by usingcoherent addition of all sub-image layers obtained from elements of acommon receive aperture, assuming the receive aperture has a width lessthan the maximum coherent aperture width. For second and third levelimage layers, many combinations of coherent and incoherent summation arepossible. For example, in some embodiments, second-level image layersmay be formed by coherently summing contributing first-level imagelayers, while third-level image layers may be formed by incoherentsumming of the contributing second-level image layers.

Time-domain frames may be formed by any level of image layer dependingon the desired trade-off between processing time and image quality.Higher-level images will tend to be of higher quality, but may alsorequire more processing time. Thus, if it is desired to providereal-time imaging, the level of image layer combination processing maybe limited in order to display images without significant “lag” beingvisible to the operator. The details of such a trade-off may depend onthe particular processing hardware in use as well as other factors.

2D Imaging while Collecting 3D Data

In some embodiments, a form of 2D imaging may be performed using a probeand imaging system configured for 3D imaging by simply beamforming anddisplaying only a 2D slice of data from the received three-dimensionalecho data. For example, such techniques may be used in order to reduce abeamform calculation and simplify display for real-time imaging using animaging device with limited processing capability, while still retainingthe full 3D echo data.

For example, a two-dimensional plane may be selected (automatically orby a user) from the voxels making up a three-dimensional volumetricrepresentation of the imaged region, voxels intersecting the selectedplane may be identified. An image window may be defined by automaticallyor manually selecting a portion of the selected plane, and atwo-dimensional image of the selected image window may then be formed bybeamforming only those echo samples corresponding to the voxelsintersecting the selected plane and lying within the selected imagewindow. The selected two-dimensional image window may then be displayedin real-time while three-dimensional data of the entire insonifiedvolume is simultaneously collected. In some cases two separate imagewindows in the same or different image planes may be defined and imagedsimultaneously from the same set of real-time three-dimensional echodata.

The collected full 3D echo data may be beamformed and reviewed at alater time using a device with greater processing power. In someembodiments, the 2D slice to be beamformed and displayed may beautomatically selected by an imaging device. Alternatively, the 2D sliceto be beamformed and displayed may be selected or adjusted by anoperator of the device.

Data Capture & Offline Analysis

In various embodiments, raw un-beamformed echo data resulting from aping transmitted from point-source transmit transducers and received byone or more arrays of receive transducer elements may be captured andstored in a raw data memory device for subsequent retrieval andanalysis. Alternately, captured echo data may be digitally transmittedover a network for remote processing, beamforming, and/or viewing. Inaddition to echo data, additional data may also be stored and/ortransmitted over a network and retrieved for subsequent and/or remoteimage generation and analysis. Such additional data may includecalibration data describing the positions of the transmitting andreceiving transducer elements, and transmit data describing the identity(or position) of transmitting transducers associated with specific echodata.

After retrieving such data, a clinician may use the data to reconstructimaging sessions in a variety of ways while making adjustments that maynot have been made during a live imaging session. For example, images ofa series of 2D slices through a 3D volume may be generated and shown insuccession in order to simulate a 2D transducer passing across a surfaceof the region of interest. Examples of these and other methods aredescribed in Applicant's US Patent Application Publication 2014/0058266,titled, “Ultrasound Imaging System Memory Architecture” and PCT PatentApplication Publication WO2016/028787, titled “Network-Based UltrasoundImaging System.”

Some embodiments of a probe configured for imaging an entire patient'sbody or a substantial portion of a patient's body may comprise an arrayof designated point-source transmitters and designated receive elementssized and arranged to cover a substantial portion of the desired regionof a patient's body. For example, a probe may be sized to coversubstantially half of a patient's chest or more. Such a probe may have amaximum dimension of about 8 cm to about 10 cm.

Alternatively, a much smaller probe capable of insonifying aconically-shaped volume of, for example, + or −30 degrees, can be placedon a patient's body such that an organ of interest may be included inthe cone. Such a probe may be placed in more than one place to cover alarger volume of interest.

Designated Point Source Transmitters

As described above, a point-source transmitter may be approximated usinga single small transducer element of a transducer array. When performing2D ping imaging using a 1D array (an array of elements with parallellongitudinal axes, typically including a lens to focus the signal into asingle imaging plane), a single element may be able to produce a pingwith sufficient energy in the imaging plane to achieve imaging at areasonable depth.

Additionally, in various embodiments, an “apparent point-sourcetransmitter” transducer may be configured to produce a waveform thatboth approximates an actual point-source and has sufficient energy toproduce high quality images at the desired depth. In some cases, suchapparent point-source transmitters may be configured such thatultrasound power output may be limited only by safety considerationswithin the imaged medium.

As used herein, the phrase “point-source” refers to a point intwo-dimensional (2D) space or three-dimensional (3D) space thatrepresents a center point of a transmitted 2D or 3D ultrasound waveform,respectively. In some embodiments, such a point is ideally an infinitelysmall point corresponding to a produced wavefront with a consistentsemi-spherical shape. In embodiments in which such a waveform isproduced by a single small element, such a point may lie on the surfaceof the transducer element. As used herein, the terms “semi-sphericalpulse” and “semi-spherical wavefront” may refer to any ultrasoundwavefront with a spherical-section shape, including wavefronts withapproximately spherical-section shapes greater than or less-than anideal semi-sphere. Similarly, the terms “semi-circular pulse” and“semi-circular wavefront” may refer to any ultrasound wavefront whichappears in an imaging plane to have a circular-section shape, includingwavefronts with approximately circular-section shapes greater than orless-than an ideal semi-circle.

In some cases, multiple (e.g., two, three, four or more) smalltransducer elements from a common transmit/receive array may be excitedsimultaneously to produce a ping with more energy than may be producedby a single element.

In some embodiments, a designated point source transmitter may comprisea large transducer shaped and configured to produce a relativelyhigh-power waveform that “appears” to have originated from apoint-source even if the location of the apparent point source does notcorrespond to a physical structure responsible for producing thewavefront—in other words, an apparent point-source. When performingbeamforming calculations to determine the location of reflectors basedon the timing of received echoes, the location of the apparentpoint-source may be used as the origin of the transmitted pingwavefront. In various embodiments, an approximate location of anapparent point source may be estimated based on a physical geometry orother characteristic of a designated point source transmitter.

In some embodiments, particularly suitable shapes for designated pointsource transmitters may include individual elements in the shape ofconcave and convex spherical caps sized to deliver sufficient ultrasoundpower to perform real-time three-dimensional ping-based imaging. Convexspherical caps may generally be referred to herein as “dome-shaped,”while concave spherical caps may be referred to as “bowl-shaped.” Someexamples of imaging probes incorporating examples of such spherical captransducer elements (otherwise referred to as “apparent point source”transducer elements) are shown and described in U.S. Patent ApplicationPublication No. US 2015/0080727.

Examples of Piezoelectric Materials and Manufacturing

In some cases, a designated point source transmitter may include asingle structure or one or more element sub-structures arranged in aplanar, convex, or concave shape. In some embodiments, each designatedtransmitter element or receive transducer elements may be constructedfrom a single contiguous piece of a piezoelectric material. Suchelements may be referred to herein as “bulk” elements or as being madefrom “bulk piezoelectric” materials. In other embodiments, transmitterand/or receiver elements may be composed of a plurality of sub-elementstructures, such as cut piezoelectric materials, micro-elementstructures (as described further below), or other structures that may beoperated collectively to form a complete element.

In some embodiments, transmitter elements or receiver elements may be inthe form of a thin shell of a piezoelectric material in a planar shapeor in the shape of a truncated spherical cap. Such elements may be madeof any material exhibiting piezoelectric properties. Many naturallyoccurring and synthetic materials are known to exhibit piezoelectricproperties that may be of a character suitable for use in ultrasoundimaging applications. In the case of ping-based multiple apertureultrasound imaging, ultrasound ping signals may be transmitted atfrequencies commonly used in diagnostic medical ultrasound, e.g., in therange of about 1 MHz to about 20 MHz or more. Thus, apparentpoint-source transducers with fundamental frequencies within this rangemay be suitable for use in ping-based multiple aperture imaging.

Naturally-occurring piezoelectric materials include quartz, topaz andtourmaline, while man-made piezoelectric ceramic materials include leadzirconate titanate (PZT), barium titanate, lead metaniobate, &polyvinylidene difluoride (PVF₂—not naturally piezoelectric, but may bemade so by heating in the presence of a strong electrical field). Someman-made piezoelectric ceramic materials may be combined withnon-piezoelectric polymer materials to create piezo-composites.

In the case of bulk elements made from piezoelectric materials, thethickness of a designated transmitter element, whether planar,bowl-shaped, or dome-shaped, may be directly related to the fundamentalfrequency of the transducer. In some cases (e.g., for some piezoelectricceramic materials), the thickness of a transducer shell may be equal toabout half a wavelength of its corresponding fundamental frequency.However, depending on the materials and/or structures used, the shellthickness may be differently related to a transducer element'sfundamental frequency. Manufacturing processes may also vary dependingon the piezoelectric material used and other factors.

For example, natural or man-made piezoelectric material may be machinedusing traditional techniques in order to form the desired shape directlyfrom a block of material. Such machining may be performed usingmechanical cutters, water jets or any other available machiningtechnique. Alternatively, a block or sheet of piezoelectric material maybe machined into a plurality of elements attached to a flexiblesubstrate which may then be formed into the desired shape. For example,a plurality of concentric ring cuts 42 and radial cuts 44 may be made ina sheet of piezoelectric material, which may then be formed over abacking material with the desired shape (e.g., a spherical-cap). In suchembodiments, the individual sub-element sections that make up theelement may be electrically connected so as to transmit simultaneouslywithout phasing.

In some embodiments, a desired shape may be molded (e.g., by injectionmolding, die casting, or other molding process) from a piezo-compositematerial. Examples of molding processes that may be adapted to formingspherical-cap elements are described in U.S. Pat. Nos. 5,340,510 and5,625,149.

Ultrasound transducers may also be produced in desired shapes usingadditive manufacturing techniques (commonly known as 3D printingtechniques). For example, US Patent Application Publication 2013/0076207and US Patent Application Publication 2013/0088122 describe systems andmethods for forming transducers in the shape of cylindrical posts.Similar techniques may also be adapted to form transducers withspherical-cap or other shapes. Additionally, other manufacturingtechniques such as laser sintering, stereo lithography, chemical vapordeposition or any other suitable techniques may be used to producetransducers in the shapes and sizes described herein.

Capacitive Micromachined Ultrasound Transducer (CMUT) formationtechniques may also be used to form transducers of desired shapes onto apre-shaped substrate. WO 2012/112540 shows and describes some examplesof structures and techniques that may be adapted to formingspherical-cap shaped transducers. Alternately, a transducer element maybe made by forming an array of CMUT transducers on a substratepre-formed into a desired shape (e.g., a concave or convex spherical capas described above). In such embodiments, the CMUT elements may beelectrically connected so as to transmit simultaneously without phasing.

In some embodiments, transducer elements may be made up of a pluralityof micro-elements which may be made using lithographic techniques,thin-film deposition techniques, etching techniques, additivemanufacturing techniques, surface micromachining, bulk micromachining,and/or other methods. For example, U.S. Pat. No. 6,222,304 to Bernstein(which is incorporated herein by reference) describes processes formaking micro-shell transducer elements which include a substrate, anelectro-active medium mounted on the substrate and including an archedsection spaced from the substrate defining a chamber between thesubstrate and arched section, and a pair of electrodes mounted on themedium for either applying an electric field across the medium forflexing the arched section or sensing the electric field generated by aflexure of the medium. Other techniques may also be used to producemicro-elements of different types. For example, in some cases, theflexible “arched section” may be replaced by a flexible planar sectionor a flexible concave section that may be flexed upon application of anappropriate electrical signal.

In some embodiments, in order to build transducer elements into a probe,the “front” surface of each element (e.g., the inside surface of aconcave element, the outside surface of a convex element or theoutward-facing surface of a planar element) may be plated with anelectrically conductive layer (such as a metal) so as to allowelectrical connection to the element. In some embodiments, the entireinner surface and the entire outer surface of each element may beplated, thereby allowing the entire piezoelectric shell to be activatedby applying an electrical signal across the two plated surfaces. Inother embodiments, less than an entire shell may be activated by platingless than an entire inner and/or outer surface of the shell. In thisway, a similar range of element sizes may be made functional from asingle physical structure.

Using materials and methods available, transmitter and receiver elementsmay be made in a range of shapes, such as having a top surface that isplanar, convex, concave, or combination of shapes. Transmitter and/orreceiver elements may also have various plan-view shapes such ascircular, rectangular, pentagonal, hexagonal, heptagonal, octagonal,other polygonal, or others. Elements can be electrically and logicallyconnected to a controller so as to be dedicated transmitters, dedicatedreceivers, or switchable between transmit and receive functions.Elements can be arranged into various arrays, such as regular arrays,irregular arrays, sparse arrays, or arrays that may be controlled so asto be operated as regular, irregular, or sparse arrays.

Ultrasound Imaging Arrays Constructed from Micro-Elements

Ultrasound probes made up of ultrasound transmitters and receivers maybe made by various techniques suitable for producing arrays ofmicro-elements. As used herein, the term “micro-element” refers to verysmall transducer elements with sizes (e.g., diameters) on the order ofless than about 1,000 microns, in some cases about 10 microns to about500 microns, in some cases between about 50 microns to about 200microns, in some cases between about 50 and about 150 microns, in somecases between about 10 microns and about 25 microns, in some casesbetween about 25 microns to about 50 microns, and in some cases betweenabout 50 microns and about 100 microns. In some cases, a micro-elementmay be defined as any element that has a diameter equal to less thanhalf a wavelength of the ultrasound frequency in the imaged material atwhich the micro-element will be used.

Typically, micro-elements may be too small to be mechanically machinedfrom traditional bulk piezoelectric materials, and may therefore be madeusing lithographic techniques, thin-film deposition techniques, etchingtechniques, additive manufacturing techniques, surface micromachining,bulk micromachining, and/or other methods. For example, U.S. Pat. No.6,222,304 to Bernstein (which is incorporated herein by reference)describes processes for making micro-shell transducer elements whichinclude a substrate, an electro-active medium mounted on the substrateand including an arched section spaced from the substrate defining achamber between the substrate and arched section, and a pair ofelectrodes mounted on the medium for either applying an electric fieldacross the medium for flexing the arched section (i.e., in order totransmit an ultrasound signal) or sensing the electric field generatedby a flexure of the medium (i.e., in order to receive an ultrasoundsignal impinging on the micro-element). Other techniques may also beused to produce micro-elements of different types. For example, in somecases, the flexible “arched section” may be replaced by a flexibleplanar section or a flexible concave section that may be flexed uponapplication of an appropriate electrical signal and/or may be sensitiveto received impulses.

In some embodiments, it may be desirable to create arrays ofmicro-elements capable of conforming to a desired shape. In suchembodiments, a pattern of micro-elements may be formed on asemiconductor substrate using one or more of the above techniques, aback-surface (i.e., opposite the surface on which micro-elements lie) ofthe substrate may be thinned to make the substrate flexible to a desireddegree, and the substrate may then be conformed to a new shape andsecured to a supporting material. In various embodiments, such thinningand re-shaping may be performed over an entire substrate surface or inlocalized regions.

There are generally four methods for wafer thinning common in the art ofsemiconductor manufacturing. They include mechanical grinding, chemicalmechanical polishing (CMP), wet etching and atmospheric downstreamplasma (ADP), and dry chemical etching (DCE).

Using such techniques (or combinations thereof), one or more sections ofan array of micro-elements may be conformed so as to form entire arraysor array sections in desired shapes. For example, arrays may be shapedso as to conform to a portion of a human anatomy, such as a breast, ajoint (e.g., a knee, elbow, wrist, ankle, etc.), a skull, or otheranatomical structure. In other examples, arrays may be shaped so as toconform to mechanical, structural, or industrial parts. In otherexamples, array sections may be shaped to form three-dimensional elementgroups arranged to exhibit desired transmit patterns. For example, suchtechniques may be used to form transducer element groups configured totransmit apparent point-source waveforms.

In some embodiments, an array of micro-elements may be conformed so asto form dome-shaped sections or dish-shaped sections, such as thespherical section elements described herein. For example, convex orconcave dome-shaped transmit elements optimized for near-field imagingmay be formed from a plurality of micro-elements on a thinned section ofsubstrate and conformed to a desired dome shape.

In some embodiments, a dome-shape may be optimized for near-fieldimaging based on transmit angles of micro-elements that will make up thedome. Each micro-element may have a transmit angle relative to acenterline of a transmitted wavefront. The transmit angle of amicro-element defines the angle at which the energy of a transmittedwavefront drops below some threshold value.

In some embodiments, a convex dome shape optimized for near-fieldimaging may be defined by a spherical section with a cut elevationselected such that wavefronts transmitted from micro-elements at a lowregion of the dome (i.e., a region of the spherical section adjacent anattachment point with the surrounding micro-element array) will not tendto impinge directly on adjacent micro-elements. The micro-elements ofsuch a dome-shaped group may be electrically connected such that theymay be operated as a single element. As in various other examplesherein, the spherical center point of such an element may be used as theacoustic center point for the purpose of beamforming calculations forpings transmitted from such a spherical-section shaped transmitterelement.

In some embodiments, a concave dish shape optimized for far-fieldimaging may be defined by a spherical section with a cut elevationselected such that wavefronts transmitted from micro-elements within thedish will not tend to impinge directly on other micro-elements withinthe dish-shaped element. The micro-elements of such a dish-shaped groupmay be electrically connected such that they may be operated together tocollectively form a single transmitter and/or receiver element. As invarious other examples, the spherical center point of such an elementgroup may be used as the acoustic center point for the purpose ofbeamforming calculations for pings transmitted from such aspherical-section shaped transmitter element or receiver element.

In some embodiments, an overall shape of a substrate supporting an arrayof micro-elements may be conformed into a desired shape. For example, insome embodiments a substrate supporting an array of micro-elements maybe formed into complex overall shapes. In some cases, an array ofmicro-elements may include one or more surfaces with inflected sections,such as to conform to an anatomical or other structure.

In various embodiments, micro-elements may be grouped to form shapedtransducer elements such as those described elsewhere herein. Groups ofmicro-elements may be treated collectively as a single element in thevarious processes described herein, and may be referred to herein as“element groups” or “micro-element groups.” For example, as shown inFIG. 1 and FIG. 2, a plurality of adjacent micro-elements may belogically and electrically treated as a single unit so as to formtransducer elements of a desired size and shape, and in a desiredlocation. In some embodiments, spherical cap or dome-shaped elements maybe formed from a plurality of micro-elements.

One advantage of forming transducer elements from a group of suchmicro-elements is that the position of each micro-element may be knownvery precisely based on the precision of manufacturing techniques usedto produce the micro-elements. Therefore, the position of eachmicro-element-group may also be known very precisely.

In some embodiments, a group of micro-elements forming a singletransducer element group may collectively share a single electricalground conductor and a single electrical signal conductor. In someembodiments, each of the ground conductor and the signal conductor foreach transducer element group may form a differential pair independentof signal and ground conductors for other elements of the array. Inother words, micro-elements in a group of micro-elements forming anelement group may share a single ground conductor that is not sharedwith ground conductors of micro-elements belonging to other elementgroups between the probe and a connector joining the probe to controlelectronics. Similarly, micro-elements of an element group may share asingle signal conductor that is not shared with signal conductors ofmicro-elements of other element groups between the probe and a connectorjoining the probe to control electronics. In some embodiments, somemicro-elements (e.g., a sub-set of micro-elements forming an elementgroup) may be joined to signal and/or ground conductors of othermicro-elements via a switch configured to allow the sub-set ofmicro-elements to be switchably joined to signal and/or groundconductors of other micro-elements.

Alternately, some or all micro-element groups may share a common groundconductor and/or a common signal conductor. For example, in someembodiments micro-element groups that form elements that arecollectively part of a single aperture may share a common groundconductor and/or a common signal conductor. In other embodiments, one ormore signal and/or ground conductors may be shared in a multiplexedarrangement, such as time-division-multiplexed communications, or othermultiplexing method.

Sparse Array Probes for Real Time 3D Imaging

Ping-based multiple aperture ultrasound imaging can provide verypowerful real-time three-dimensional imaging capabilities as describedabove. The benefits of ping-based multiple aperture ultrasound imagingmay be achieved by using transducer probes with overall dimensions muchlarger than traditional ultrasound probes. For example, ping-basedmultiple aperture ultrasound imaging may be beneficially used withprobes having an active imaging surface in excess of 100 cm².Traditionally, ultrasound elements in a probe are spaced as closetogether as possible, typically significantly less than (and generallyno more than) half a wavelength of the ultrasound frequency being used.

However, using traditional element-to-element spacing in such a largeprobe would require a cable far too thick for the cable to be usable.Although some tricks may be used to reduce the number of individualconductors required in a cable, a better solution is to increase theallowed spacing between elements, thereby reducing the total number ofelements in an array. Use of sparse arrays with traditionalscanline-based imaging methods suffers from substantial complications,artifacts, and low resolution and is therefore not generally practical.Based on the research into the use of sparse arrays scanline-basedphased array techniques, one would expect the use of sparse arrays withping-based multiple aperture ultrasound imaging techniques to suffersimilar difficulties, but that is unexpectedly not the case. In fact,sparse arrays can be used quite effectively with ping-based multipleaperture ultrasound imaging techniques as described herein.

In some embodiments, sparse arrays of transducer elements may bebeneficial in providing an ultrasound probe with a wide total aperturewhile containing a manageable number of transducer elements.

In other fields, a “sparse array” is generally defined as an array inwhich the majority of array positions have a zero or null value. In somecases, a similar definition may be applied to an array of ultrasoundtransducer elements. In the context of an array of ultrasound transducerelements, a sparse array may be defined as an array of potential elementpositions in which a majority of the element positions contain no activeelements. For example, the inactive element positions may contain notransducers, or may contain transducers that are not active at aspecified time, if ever. For example, inactive element positions maycomprise transducer elements that are temporarily or permanentlyelectrically disconnected from imaging control electronics. In otherexamples, inactive element positions may comprise empty spaces (orspaces filled with non-transducing material) of sizes equivalent to atransducer element.

In some cases, an ultrasound transducer array in which less than amajority of element positions contain no active elements may also beconsidered “sparse” if average spacing between adjacent elements exceedsa threshold distance. For example, in some cases, an ultrasoundtransducer array may be considered “sparse” if all (or nearly all)adjacent elements of the array are spaced from one another by a distanceof at least half a wavelength of the ultrasound transmitted and/orreceived by the elements. In still other cases, an ultrasound transducerarray may be considered “sparse” if at least a majority of elements ofthe array are spaced from adjacent elements by a distance of at least athreshold distance. If an array is configured to operate at more thanone ultrasound frequency, then a threshold distance may be half of themaximum wavelength at which any part of the array is configured tooperate.

In some various, ultrasound probes comprising a sparse array ofultrasound transmitters and receivers may be made using one or more ofthe various micro-element or sub-element configurations describedherein. For example, in some embodiments, a sparse array may be formedfrom a continuous array of micro-elements by electrically assigningmicro-elements to element-groups where the element groups are located ina sparse arrangement. Examples of such embodiments are described belowwith reference to FIG. 1. In some such embodiments, micro-elements thatare not assigned to an element-group may simply be electricallyinactive.

In some embodiments, instead of a continuous array of micro-elements (asin FIG. 1), a sparse-array transducer probe may comprise a plurality ofgroups of micro-elements formed in desired locations on a substratewithout necessarily forming additional micro-elements betweenmicro-element groups. FIG. 2 illustrates an example of such a sparsearray 480 made up of separated micro-element groups 481-486.

In some embodiments, ultrasound probes comprising a sparse array ofultrasound transmitters and receivers may be made by forming individualtransducer elements from bulk piezoelectric material, mechanicallypicking up and placing each element in a precise location on asubstrate, securing each element to the substrate, and making electricalconnections to each element. Bulk piezoelectric elements may be made inany shape or size as described herein.

In some embodiments, a sparse array probe may be effectively used by 3Dmultiple aperture imaging techniques as described herein, and in somecases may be preferred to a continuous array densely populated withtransducer elements, particularly when spacing between receivers andtransmitters avoid certain patterns. In certain circumstances, using aprobe having a sparse two-dimensional or three-dimensional array ofregularly-spaced transducer elements with the ping-based multipleaperture imaging beamforming techniques described herein may result inthe production of self-reinforcing artifacts that may undesirablydegrade image quality. Such artifacts may be the result of phasecancellation causing certain returning echo signals to cancel oneanother out before reaching some receive elements thereby producingdistortions in the form of extremely dark and/or extremely bright bandsin the resulting image.

In some embodiments, such artifacts may be avoided or mitigated bypositioning transducer elements at inconsistent distances from oneanother in a sparse array. In other words, phase cancellation artifactsmay be avoided or mitigated by positioning transmit and receive elementssuch that no two transducer elements are an equal distance from anysingle third element. In some cases, a small amount of element positionequivalence may be acceptable. In examples of such embodiments, a probemay be constructed such that no more than “N” receive elements arelocated an equal distance from a single transmitter, where “N” is aninteger value between 1 and 1,000 or more, more commonly between about 1and 100, and in some specific examples N may be 1, 2, 3, 4, 5, 10, 100or more and wherein all of the receiver elements receive the samefrequency of ultrasound signal.

In some embodiments, transmit elements may be regularly-spaced relativeto one another while spacing receive elements at irregular distancesrelative to the transmit elements. In other embodiments, both transmitelements and receive elements may be irregularly positioned in a sparsearray.

In some embodiments, a two-dimensional sparse array of transducerelements positioned to mitigate phase cancellation artifacts may includeelements spaced from one another at “random” distances. FIG. 3 and FIG.4 illustrate examples of two-dimensional sparse array element positionsin which a distance between any two adjacent elements is not exactlyequal to any other distance between two adjacent elements. In theexample of FIG. 3 and FIG. 4, the distances between adjacent elementsare effectively random in the sense that there is no mathematicalpattern relating the distances.

FIG. 3 illustrates a two-dimensional array 401 made up of elements 410unevenly spaced from one another in two dimensions according to apseudo-random pattern. In various embodiments, the array 401 illustratedin FIG. 3 may have an overall length dimension 413 and/or an overallwidth dimension 414 of about 3 cm to about 10 cm or more. Each element410 of the array 401 may have a square shape, a circular shape, apolygonal shape, or any other regular or irregular shape. For example,FIG. 4 illustrates a sparse array 402 embodiment in which designatedtransmit elements are shown as circular elements 411 while designatedreceive elements are shown as square elements 412. In other embodiments,all elements may be circular, square, or otherwise-shaped elements evenif some are designated transmitters and others are designated receivers.

In addition to the generally rectangular arrays shown in FIG. 3 and FIG.4, sparse arrays 401, 402 of ultrasound elements with uneven spacing maybe arranged in other shapes, such as a generally oval shaped array 403as shown in FIG. 5, or a generally circular shaped array 404 as shown inFIG. 6.

Sparse arrays of ultrasound elements with uneven spacing may also bearranged in a generally planar configuration, or a generally concave orconvex configuration 405 as shown in FIG. 7. The arrangement oftransducer elements shown in FIG. 7 may be made convex by a supportingsubstrate layer positioned on the concave side of the pattern ofelements. Similarly, supporting the elements with a substrate on theconvex side of the pattern will provide a concave array. In otherembodiments, sparse arrays may be arranged for use in intra-venousultrasound probes or other specialized probes.

In some embodiments, spacing distances between elements may benon-repeating random or pseudo-random distances obtained with use of arandom or pseudo-random number generation algorithm. In other examples,spacing distances between elements may be irregular values which may bebased on non-repeating values from an integer sequence such as theFibonacci sequence or any other non-repeating numeric sequence. In somecases, an algorithm may be applied to values from a numeric sequence inorder to maintain element spacing distances within a desired range. Forexample, in some embodiments distances between adjacent elements may beconstrained to a range such as 1 mm to 10 mm (or 1 mm to 20 mm or more).

In various embodiments, the spacing between transducer elements needonly be un-equal by an amount at least as great as a manufacturingtolerance of a manufacturing method used to construct the array. Forexample, if an array manufacturing process is capable of positioningelements on a substrate within a distance of +/1 100 microns of anintended position, then it may be desirable to design the two spacingdistances so as to be different by at least 100 microns. In otherembodiments, the spacing between transducer elements need only beun-equal by an amount based on a frequency of ultrasound used.

In some medical imaging applications, variations in tissues mayintroduce enough randomness to signals to substantially avoid orminimize most phase cancellation artifacts. Therefore, in some cases asparse array probe may include a larger number of regularly-spacedtransducer elements if other factors can be expected to minimize phasecancellation artifacts. For example, FIG. 8 illustrates an example array302 of dedicated transmit elements 310 and receive elements 312. Invarious embodiments, the transmit elements 310 and/or the receiveelements 312 may be replaced with any other transmit or receive elementsdescribed elsewhere herein. FIG. 15 and FIG. 16B also illustrateelements with examples of regularly spaced elements which may containelements of any construction such as those described herein.

Multiple Frequency Sparse Arrays

In some cases, phase cancellation artifacts may be avoided or mitigatedby constructing and/or operating a sparse array probe such that only asmall number of regularly-spaced elements operate at the same ultrasoundfrequency as one another. For example, some embodiments of sparse arrayprobes may be constructed and/or operated such that no more than “N”receive elements operating at the same ultrasound frequency (orcombination of frequencies) are equidistant to any one transmit element.In such embodiments, N may be an integer between 1 and 100, in somespecific examples, N may be 1, 2, 3, 4, 5 or more. Thus, for example, aprobe may contain any number of receive elements equidistant to one ormore transmit elements as long as no more than N of the receivers areoperated at the same ultrasound frequency (or combination offrequencies).

The ultrasound frequency (or combination of frequencies) at which anygiven transmit or receive element may be operated may be based onstructural characteristics of the element (e.g., material, thickness,diameter, or other dimensions) and/or variable operationalcharacteristics such as an electric voltage or signal shape applied tothe element. Which and to what degree such factors may change anelement's operating frequency may depend on the element material and/orthe process by which it is manufactured. For example, while manytransducers may have a fundamental frequency, many can also be driven(i.e., operated in transmit or receive) at frequencies other than theirfundamental frequency. The range of frequencies at which any particulartransducer element may be driven may depend on many factors such as thematerial, construction, available power, etc.

For example, in various embodiments, an operating frequency of amicro-element may be determined by a diameter of a flexible section ofmaterial, such as the “arched section” described by Bernstein referencedabove or a similarly-configured flexible planar or concave section.Thus, in some embodiments, micro-element groups may be made up entirelyof micro-elements configured to operate at the same frequency, or may bemade up of micro-elements configured to operate at differentfrequencies. For example, in some embodiments an outer micro-elementgroup (e.g., 464 in FIG. 1) may be configured to operate at a different(e.g., a higher frequency or a lower frequency) than micro-elementsmaking up a central group 466, either by being physically constructeddifferently or by being operated differently.

FIG. 9 illustrates an example of a group of micro-elements of differentsizes arranged to form a multi-frequency transmit element. As describedabove, the fundamental transmit frequency of a micro-element may be afunction of the size of a flexible surface member. Therefore, amulti-frequency transmit element group may be formed by providingmicro-elements of varying sizes arranged so as to be controllable as acommon element group. The group 500 of micro-elements in FIG. 9 includesmicro-elements of three different sizes. In other embodiments,micro-elements of two, four, five, six, or more different sizes may begrouped into a transmitter or receiver group. Other patterns or numbersof micro-elements of each size may also be used, depending on thewaveform characteristics desired.

The variously-sized micro-elements of FIG. 9 may be electricallyconnected so as to be activated simultaneously as a group, such as byproviding a single electrical signal conductor and a single electricalground conductor common to all of the micro-elements of a transmit groupsuch as that shown in FIG. 9.

In some embodiments, a micro-element array may comprise a plurality oftransmit element groups containing micro-elements of various sizes. Insome embodiments, different transmit elements may be provided withdifferent mixes of micro-element sizes in order to produce pings withdifferent multi-frequency combinations. In this way, each transmit groupmay have a unique frequency signature. If different transmitters in aprobe have different mixes of micro-element sizes to produce a differentfrequency signature, then pings transmitted from one transmitter may bedistinguished from pings transmitted by a second transmitter, even ifpings from the two transmitters are transmitted at the same time orduring overlapping ping cycles.

In other words, when a probe is configured to include at least twotransmitters configured to transmit multi-frequency waveforms andreceive elements of the probe are sensitive to all of the transmittedfrequencies, then echoes received by each receive element of the probemay be mapped to the transmitter that produced the echo based only onthe frequency signature of the received echoes. This may be tremendouslybeneficial in increasing ping rates and/or frame rates well beyond thelimits imposed by single-frequency imaging.

Sparse Arrays with Varied or Variable Element Sizes

In some embodiments, a size of transducer elements may be varied insteadof or in addition to varying spacing between elements. For example, insome embodiments, a sparse ultrasound array may be entirely made up oftransducer elements of various (and/or variable) sizes as shown forexample in FIG. 1, FIG. 10A, FIG. 10B, FIG. 11A, FIG. 11B.

In some embodiments, micro-element groups may be switched between afirst configuration in which a first element group includes a firstgroup of micro-elements and a second configuration in which the firstelement group includes a second group of micro-elements in addition to(or subtracted from) the first group of micro-elements. In someembodiments, such switching between a first configuration and a secondconfiguration may be performed in between ping cycles. That is, a firstping may be transmitted and echoes of the first ping may be received bythe micro-elements of the first configuration of the element group.Then, a second ping may be transmitted, and echoes of the second pingmay be received by the second configuration in which the first elementgroup includes a second group of micro-elements in addition to the firstgroup of micro-elements.

Similarly, a transmit element group may be configured to be switchablebetween a first configuration and a second configuration that is larger,smaller, or differently shaped than the first configuration.

Alternatively, element groups may be switched between a firstconfiguration and a second configuration within a single ping cycle. Insuch embodiments, a first ping may be transmitted, a first plurality ofechoes of the first ping may be received by the micro-elements of thefirst configuration of the element group, and the switch may then beclosed so that a second plurality of echoes of the first ping may bereceived by the micro-elements of the second configuration of theelement group in addition to or instead of the micro-elements of thefirst configuration. In some embodiments, the first plurality of echoesmay be relatively near-field echoes that are produced by reflectorscloser than a threshold distance to the receiver micro-elements, and thesecond plurality of echoes may be mid-field or far-field echoes that areproduced by reflectors further than a threshold distance from thereceiver micro-elements. Assuming an approximately constant speed ofsound, the threshold may be a time value, such that the first pluralityof echoes may be received with the first configuration of micro-elementsuntil a switch-time at which a switch may activate the secondconfiguration of micro-elements to allow the second plurality of echoesto be received after the switch-time until the end of the ping cycle.

In some embodiments, individual micro-elements or groups ofmicro-elements may be electrically switchable so as to be selectivelyincluded in or excluded from a micro-element group forming an element oran aperture. Transmit element groups or receive element groups may beconfigured to be switchable between a first configuration and a secondconfiguration, where the second configuration is larger, smaller, ordifferently shaped than the first configuration.

For example, FIG. 1 illustrates an example section of a continuous array450 of micro-elements 460. Some of the micro-elements 460 are indicatedas having been assigned to an element-group 462, 464, 466, 468, 470, and472. FIG. 1 further illustrates an outer group of micro-elements 464identified by stippling surrounding a central group 466 ofmicro-elements. In one embodiment of the illustrated example, a largerelement group 468 may be used as a transmit element, and the smallerelement groups 462, 464, 470, and 472 may be used as receive elements.

In some embodiments, the outer group 464 may be switchable as a group soas to selectively form a larger element in combination with the centralgroup 466. Similar element groups may be formed with any number ofadjacent micro-element groups in any desired configuration. Such avariably-sized element may be used as a transmit element, as a receiveelement, or as both.

Another example is provided in the groups 472 and 474 which provide forswitchable configurations with different shapes and sizes. One or moreswitches may be provided to allow the group 474 to be included with thegroup 472 so that both groups 472 and 474 may operate together as asingle element (e.g., as a receive element or as a transmit element).The elongated element formed by the combination of sub-groups 472 and474 may be beneficially used in combination with the process forestimating a position of a reflector using an elongated receive element.

In some embodiments, ping-based receive beamforming calculations (asdescribed herein) may be performed using the position of the circularcenter of the center micro-element (e.g., 744) of a micro-element groupas the acoustic center position of the transducer element group. Inembodiments in which a micro-element group is arranged such that it doesnot include a centrally-located micro-element, the position of themicro-element group may be defined at a center-of-mass point or ageometric center point of the group of micro-elements. In otherembodiments, various calibration processes may be used to measure,determine, and/or to refine an acoustic center position for eachtransducer element group. Examples of suitable calibration processes aredescribed in US Patent Application Publication US 2014/0043933 titled“Calibration of Multiple Aperture Ultrasound Probes,” U.S. Pat. No.9,282,945 titled “Calibration of Ultrasound Probes,” and U.S. Pat. No.9,510,806 titled “Alignment of Ultrasound Transducer Arrays and MultipleAperture Probe Assembly,” each of which is incorporated by referenceherein.

FIGS. 10A and 10B illustrate examples of radially symmetrical groups 342of switchable concentric sub-elements 344, 346, 348 which may beoperated in concert with one another. The group 342 may include acentral circular receive element 344 surrounded by one or moreconcentric-ring receive elements 346, 348 providing the ability toobtain the benefits of elements of various sizes in a small physicalfootprint while maintaining a consistent acoustic center position. Invarious embodiments, a center element 344 may be surrounded by two,three or more rings, depending on the needs of a particular application.

In some embodiments, the space (or “kerf”) between the center circularelement 344 and the inner ring 346 and the space/kerf between rings 344,348 may be as small as possible so as to provide as much of a seamlesstransition between adjacent ring elements as possible. In one example,the inner circular element 344 may have a diameter of about 1 mm, theinner ring 346 may have a width of about 0.5 mm, and the outer ring 348may have a width of about 0.5 mm. In such an example, the center element344 and inner ring element 346 may be combined to mimic a circularelement with a diameter of about 2 mm, and the center element 344, innerring element 346, and outer ring element 348 may be combined to mimic acircular element with a diameter of about 3 mm.

As shown in FIG. 10A, in some embodiments, each element 344, 346, 348may be individually connected to a separate receive channel 331, 332,333 of a receive subsystem so as to allow echoes received with eachconcentric element to be stored separately. Using such an arrangement, acomplete image may be formed from received echoes after echoes have beenreceived with all receive elements of all sizes. This may allow for a“digital switching” process in which echo data received with thesub-elements may be selectively combined to improve a final image.Stored echoes may be retrieved from the memory device and combined so asto obtain an optimal image based on the timing of received echoes and/orthe location of the transmitted ping.

In such embodiments, echo data received with the center element 344alone may be used for beamforming near-field reflectors. Echo datareceived with the center element 344 may be coherently combined withecho data received with the inner ring element 346 for beamformingmid-field reflectors. Similarly, echo data received with the centerelement 344 may be coherently combined with echo data received with theinner ring element 346 and the outer ring element 348 for beamformingfar-field reflectors.

As with other embodiments using receivers of various sizes, thetransitions between “near-field,” “mid-field,” and “far-field” (orbetween “near-field” and “far-field” for systems with only two sizes ofreceive elements) may be defined based on optimal characteristics of theintended imaging application, the particular sizes of the elements to beused, transmitted ultrasound frequencies, and other factors. Oneadvantage of the digital switching methods described herein is that thetransition between “near-field,” “mid-field,” and “far-field” may bechanged and redefined after echo data has been received and stored. Thismay allow for iterative adjustment of such transitions for optimizationof image quality or other desired characteristics.

Alternatively, as illustrated in FIG. 10B, each group of three ringsections 342 may be electrically connected to a common receive channelof a receive subsystem so as to allow for electrical switching between“small,” “medium,” and “large” receive elements. When the three ringsections 344, 346, 348 are arranged in a concentric pattern, thelocation of the circular center of each of the sections 344, 346, 348will be the same, thereby simplifying beamforming operations.

As illustrated in FIG. 10B, a center circular element 344 and one ormore concentric ring sections 346, 348 may be electrically connected toa common receive channel 330 via switches 352, 354. The switches 352,354 may be any remotely operable electrical switch, and may include anysuitable electromechanical, MEMS, semiconductor or other components. Thesub-elements 344, 346, and 348 may be electrically connected in parallelwhen the switches 352, 354 are closed. In some embodiments, eachsub-element 344, 346, 348 may have an independent ground conductor. Inother embodiments, the sub-elements 344, 346, 348 may share a commonground conductor.

In use, a ping may be transmitted from a transmit element at atime=“to,” and the transition between “near-field,” “mid-field,” and“far-field” may be defined in terms of time, where “t₁” is the time atwhich the transition from near-field to mid-field occurs and “t₂” is thetime at which the transition from mid-field to far-field occurs. In thisexample, both switches 352, 354 may be open during and immediatelyfollowing transmission of a ping at time t0. Then, at time t1, theinner-ring switch 352 may be closed, thereby electrically combining thesignal generated by the inner circular element 344 with the signalgenerated by the inner-ring transducer element 346. At time t2, theouter-ring switch 354 may also be closed, leaving the inner-ring switch352 also closed, thereby electrically combining the signal generated bythe inner circular element 344 with the signal generated by theinner-ring transducer element 346 and the outer-ring transducer element348. The resulting echo data string produced by the ping transmitted att0 will then contain near-field echoes received by the inner circleelement 344 alone, mid-field echoes received by the combined innercircular element 344 and the inner ring element 346, and far-fieldechoes received by all three 344, 346, 348 elements combined.

As with other embodiments described herein, receive elements of varioussizes may be made using any suitable manufacturing process or processes.For example, continuous circular disc-shaped elements and ring-shapedelements may be made of a bulk PZT material, other piezoelectricmaterials, or from arrays of micro-elements or other sub-elements usingany of the various manufacturing techniques identified herein. Thetransducer elements 344, 346, 348 may also have shapes other thancircular, such as polygonal or amorphous shapes. Each of the elements344, 346, 348 may also be made up of multiple micro-elements.

In some embodiments, a center element and one or more concentric ringelements may be formed from a single continuous piece of PZT (or otherpiezoelectric material) that is plated with an electrically conductivematerial to form a center element 344 and one or moredistinctly-operable rings 346, 348. For example, the ring elements maybe defined by rings of plated electrically conductive material in theshape of the desired ring(s) with regions of un-plated material inbetween adjacent ring elements or between an inner ring and a centerelement. In some embodiments, ring-shaped plated regions may be formedon both a top and a bottom surface of the piezoelectric material. Inother embodiments, ring-shaped sections may be plated on a top surface,and a bottom surface may be continuously plated. In some embodimentsusing such a continuous piezoelectric structure with plated andun-plated regions, a bias voltage may be applied to an inner ring 346and/or an outer ring 348 while receiving echoes with a center element inorder to dampen unwanted oscillation of the outer region of PZT.

In some embodiments, a switchable group of sub-elements such as thatillustrated in FIG. 10B may be used as a transmit element of variablesize. For example, differently-shaped waveforms or waveforms ofdifferent power levels may be produced by transmitting from the centerelement 344 alone than by transmitting simultaneously from both thecenter element 344 and the inner ring element 346, which will in turn bedifferent than a waveform transmitted from all three sub-elements 344,346, 348.

In some embodiments, the center element 344 may also be switchablyconnected to a receive system or a transmit system. In some embodiments,the arrangement illustrated in FIG. 10A may be combined with thearrangement in FIG. 10B. That is, each sub-element may be switchablebetween one of three states: connected to its own receive (or transmit)channel, connected in electrical parallel with one another, ordisconnected (i.e., at open circuit relative to the other sub-elements).

In some embodiments, elements of different sizes may be grouped indifferent constellations allowing for a different mode of operation.FIG. 11A illustrates elements of multiple sizes arranged in a group 322.In some embodiments, elements of varying sizes may be grouped together,each group 322 including a small element 324, a medium element 326, anda large element 328 positioned close together. In some embodiments, asparse array may comprise groups of elements of different sizes in whichthe elements in each group may be spaced from one another by a distanceof less than half a wavelength of the ultrasound transmitted and/orreceived by the elements. In other words, groups 322 of elements may besparsely positioned relative to other groups, while the elements of eachgroup may be non-sparsely spaced from one another.

In one example, the small element 324 may have a diameter of about 1 mm,the medium element 326 may have a diameter of about 2 mm, and the largeelement 328 may have a diameter of about 3 mm (any other sizes may alsobe desired, depending on the needs of a particular application). Inother examples, an element constellation 322 may include receiveelements of only two sizes or elements of four or more sizes. Theindividual and relative element sizes used and the relative positions ofelements may also be varied depending on the needs of a particularapplication. In some embodiments, each element of a constellation group322 may comprise concentric elements as described above with referenceto FIG. 10A and/or 10B.

In some embodiments, each element 324, 326, 328 of the constellation ofFIG. 11A and the constellation of FIG. 11B may be a micro-element (e.g.,a micro-dome as described above or another micro-element structure) or agroup of micro-elements.

As described above, smaller-diameter elements may provide optimalreceive characteristics for echoes returned by relatively shallow (ornear-field) reflectors, while larger-diameter elements may provideoptimal receive characteristics for echoes returned by relatively deep(or far-field) reflectors. Therefore, in some embodiments, an imagingsystem may be configured to switch between using information fromelements of various size such that small elements 324 may be usedprimarily for forming images from echoes received from near-fieldreflectors, medium-sized elements 326 may be used primarily for formingimages from echoes received from mid-field reflectors, and largeelements 328 may be used primarily for forming images from echoesreceived from far-field reflectors.

In some embodiments, this switching may be accomplished digitally byforming a complete image from received echoes after echoes have beenreceived with all receive elements of all sizes. To achieve this digitalswitching in some embodiments, each of the receive elements 324, 326,328 of the constellation of FIG. 11A may be individually electricallyconnected to a separate channel 331, 332, 333 of a receive subsystem. Insuch embodiments, echoes received by the elements of varying sizes maybe digitized and stored in a memory device separately. Stored echoes maythen be retrieved from the memory device and combined so as to obtain anoptimal image based on the timing of received echoes and/or the locationof the transmitted ping.

Because each received echo sample can be mapped to a three-dimensionalposition within the imaged volume, one or more threshold depths may beestablished in order to determine which regions of a volume (or 2Dimage) should be formed with echoes received by small elements, whichregions should be formed with echoes received by medium-sized elements,which regions should be formed with echoes received by large elements,and which regions should be formed by combining echoes from small andmedium elements or by combining echoes from medium and large elements.Such information can be stored in an imaging controller and used duringlive imaging or during reconstruction of an image from stored echo data.As with other embodiments described herein, receive elementconstellations may be grouped into receive apertures, each receiveaperture having an overall size selected such that echoes received bymultiple elements of a common receive aperture may be combinedcoherently without phase cancellation. Information obtained from two ormore receive apertures may then be combined incoherently.

The digital switching method described above relies on the echoesreceived by each receive element of each size being individually storedat least temporarily. In alternative embodiments, the number of receivechannels used by receive elements of various sizes may be reduced byincorporating switches. F or example, as illustrated in FIG. 11B, areceive group 322 including a large element 328, a medium element 326,and a small element 324 may be electrically connected to a singlereceive channel 330 of a receive subsystem via switches S1, S2, S3. Eachof the large 328, medium 326, and small 324 elements may be switched“on” (closed-circuit) during times when that element is expected toreturn usable (or beneficially-contributing) information. During timeswhen an element of a particular size is not expected to return usableinformation, the element may be switched “off” (open-circuit) in favorof switching on an element of a different size.

Switching of receive elements will be described with reference to anexample. Assume that a ping is transmitted from a transmit element at atime=“t₀.” If the transition between “near-field,” “mid-field,” and“far-field” is defined in terms of time, where “t₁” is the time at whichthe transition from near-field to mid-field occurs and “t₂” is the timeat which the transition from mid-field to far-field occurs, then asingle group of three differently-sized receive elements may be switchedas follows: only the small element 324 is switched on (e.g., S1 isclosed, S2 and S3 are open) from time t₀ to t₁, only the medium element326 is switched on (S2 is closed, S1 and S3 are open) from time t₁ tot₂, and only the large element 328 is switched on (S3 is closed, S1 andS2 are open) from time t₂ until the next ping is transmitted (or untilsuch time as all receivable echoes can be expected to have returned). Insome embodiments, two or more of the switches S1, S2, S3 may be combinedinto a single multi-position switch.

In some embodiments, when using differently-sized receive elements incombination with digital or electrical switching, an image formationsubsystem may use a physical location of each individual element 324,326, 328 to identify echo samples corresponding to a particular pixel orvoxel location within an imaged region. In the case of digitalswitching, the position of the circular center of each receive elementmay be individually stored and associated with the corresponding receivechannel for use during beamforming operations. Even in the electricalswitching case, because the time at which switching occurs is known, thesamples corresponding to the small, medium, and large elements can bedetermined based on times associated with the data samples, and theappropriate element circular center position information may be used forbeamforming echoes received by the elements of different sizes.

In some embodiments, transducer elements of different sizes in a patternsuch as that shown in FIG. 1, FIG. 11A, FIG. 11B, FIG. 10A, or FIG. 10Bor any other pattern, may be used as transmit elements. For example,planar, concave, or convex elements of any circular, polygonal, or othershape may be provided in two, three, or more different sizes for use asdedicated transmit elements.

In various embodiments, switches used for switching individualmicro-elements or groups of micro-elements may includemicroelectromechanical systems (MEMS) switches of any suitable type. Insome embodiments, MEMS switches may be formed on an opposite side ofsame substrate as the micro-elements. MEMS or other switches may becontrollable by transmit subsystems, receive subsystems, or both, asappropriate for a given application.

In some embodiments, low noise amplifiers (LNAs) may also be provided onthe back-side of a substrate supporting an array of micro-elements. Insome embodiments, one LNA may be provided for each receive element (orgroup of micro-elements controlled collectively). In other cases, oneLNA may be provided for each receive aperture or group of receiveelements. In various embodiments, LNAs may also be controlled by one ormore elements of an imaging control system, such as a transmit subsystemor a receive subsystem.

Detecting Reflector Position Based on Pattern or Shape of ReceiveElements

As described herein, beamforming calculations produce a locus ofpossible locations for a reflector based on a known position of atransmitter element and a receiver element, and the loci obtained frommultiple elements are combined to converge towards an actual locationfor a given reflector. Therefore, any additional information about thelikely position of reflectors may be used to further enhance imagequality.

In some embodiments, a known pattern of receiver elements may be used todetermine an approximate direction from which echoes return to thereceiver elements. This may be understood with reference to theillustration in FIG. 12, which shows a pattern 600 of six receiveelements 602, each made up of a plurality of micro-elements. Theelements 602 are shown arranged in a regular grid pattern for clarity ofillustration, but may be irregularly spaced and may be asymmetricallyaligned relative to one another. Various axes may be drawn through anytwo or more transducer elements arranged in the pattern. FIG. 12illustrates a vertical axis 610, a horizontal axis 606, and two diagonalaxes 604, 608. Several other axes could also be drawn, any of whichcould be used with the same methods.

For echoes of a given reflector arriving at the receive elements 602,the beamforming process will determine the locus of possible locationpoints for the reflector based on the position of the transmitter andthe positions of each of the receive elements 602. In some embodiments,the system may also compare the absolute time-of-arrival of the echoesof the given reflector at elements along one or more of the axes (604,606, 608, 610). For example, if the upper-right element along thediagonal axis 604 receives an echo of the given reflector at an earliertime than the same echoes arrive at the center element (a time measuredin nanoseconds), then it may be reasonable to conclude that thereflector is located in a portion of the region of interest closer tothe upper-right quadrant of the array section.

This location estimate information may be supported or confirmed bycomparing the arrival time of the given reflector echoes at theupper-right element with the arrival time of the same given reflectorechoes at the lower left element. Additional elements along the sameaxis may also be used to further confirm an estimate of an origin of thereflector. Comparing arrival times of the same given reflector alongother axes may provide further information about the approximatelocation of the reflector.

In some embodiments, a process of axis-based direction estimation mayinclude: transmitting an unfocused ultrasound ping from a transmitterapproximating a point source into an object to be imaged. Echoes of thetransmitted ping may then be received at a first receive element and asecond receive element, where a line between the first receive elementand the second receive element defines an axis. The first receiveelement and the second receive element may be located at known positionsrelative to a common coordinate system (e.g., based on position dataretrieved from a data store). The process may proceed by identifying afirst echo sample corresponding to a first reflector received at thefirst element, and identifying a second echo sample corresponding to thesame first reflector received at the second element. A firsttime-of-arrival may be determined for the time at which the first sampleecho was received at the first receive element. The firsttime-of-arrival may be based on explicit or implicit timing informationin the stored echo data. Explicit timing information may includespecific clock times recorded along with each received echo sample.Implicit timing information may include a known sample rate and a sampleposition (or interpolated sample position) of a particular samplerelative to some baseline (e.g., a start-of-ping time). A secondtime-of-arrival may be determined for the time at which the secondsample echo was received at the second receive element. The first andsecond times-of-arrival may then be compared to determine which of theelements first received the echo sample corresponding to the firstreflector. The element that received the first reflector's echo samplefirst is closest to the reflector along the axis. This information maythen be used for other elements along the same axis.

Based on the estimated given reflector position information obtainedabove, echoes of the given reflector received by elements that arefurther away from the estimated reflector position may be weighted lowerthan echoes of the same reflector received by elements closer to theestimated position of the reflector. It the first reflector is closer tothe first receiver element along the axis, then the echoes of the firstreflector received by the first element may be weighted higher than theechoes of the first reflector received by the second element when theechoes are combined to form an image. The same information may also beused to weight echoes of the same first reflector received by otherelements along the same axis. That is, echo contributions from receiveelements determined to be closer to the reflector along the axis may begiven more weight (i.e., a larger weighting factor) than echocontributions from receive elements further away from the reflectoralong the axis.

In some embodiments, one or more asymmetrically-shaped receive elementsmay be used to estimate an approximate location of a reflector in orderto improve image quality. For example, FIG. 13 illustrates receivetransducer element 650 with a generally elliptical shape having a longaxis 654 and a short axis 652. Echoes with strong directional componentsalong one or both axes of such an asymmetrical element will producerecognizable phase patterns due to the asymmetry. The different phasepatterns may be due to phase differences of echoes arrivingpredominantly along each axis.

For example, assume an echo returning to the element 650 with a strongcomponent along the long axis 656 arrives at the top point 656 at afirst time, then arrives at the center point 658 at a second time afterthe first time, and finally arrives at the bottom point 660 at a thirdtime after the second time. The echoes of a single reflector arriving tothe upper point 656 will be slightly out of phase with the echoes of thesame reflector arriving at the middle 658 and lower points 660. In thesame way, echoes arriving at different times at different points alongthe short axis 652 may also exhibit a unique phase pattern.

For a given shaped asymmetrical receive element, the phase pattern alongeach axis can be calibrated by transmitting pings from known transmitpositions relative to the asymmetrical receive element and measuring thephase response. Then the approximate direction of each arriving echo canbe estimated based on the phase pattern response of the asymmetricalreceiver(s). The calibration set of various phase patterns correspondingto various origin points may be stored, and used during imaging toestimate the approximate location of reflectors by comparing echo phasepatterns with the calibration set phase patterns.

In various embodiments, a wide range of asymmetrical receive elementshapes may be used to estimate approximate reflector locations. Forexample, any shape with at least one generally long axis and onegenerally shorter axis may be used. Such shapes may include elongatedirregular polygons such as rectangles or rectangles with roundedcorners, oblong shapes, oval shapes, or generally elongated amorphousshapes.

In various embodiments, each receive aperture may include only one, two,three, four, five, or more asymmetrical receivers. In other embodiments,an entire array of receive elements may be asymmetrical. Asymmetricalelements need not all be aligned with long axes in the same direction asone another, and in some embodiments it may be desirable to provideasymmetrical elements with long axes perpendicular to one another.

In various embodiments, asymmetrical transducer elements may be formedof any transducing material and using any manufacturing process,including those described elsewhere herein. For example, asymmetricalelements may be formed from a group of micro-elements in a micro-elementarray.

Although the examples of shaped receive elements above are describedrelative to arrays made up of micro-elements, the same techniques andprinciples may also be applied using elements of different constructionsmade by various other manufacturing processes. For example, thetechniques may also be applied using machined bulk PZT elements.

Sparse Arrays with Overlapping Micro-Element Groups

In some embodiments, it may be desirable to configure a ping-basedultrasound imaging probe with dedicated transmitter elements and receiveelements grouped into overlapping receive apertures in a constellationconfiguration. For example, FIG. 14 illustrates an array 700 ofmicro-elements 710 in which micro-elements are grouped into transmitelement groups and receive element groups.

As shown, each transmit element group 702 (indicated by “X” hatchedmicro-elements) may be surrounded by a plurality of receive elementgroups 704 (indicated by “/” hatched micro-elements). In variousembodiments, receive elements 704 may be grouped into receive apertures706 based on their proximity to a transmit element 702. Receiveapertures 706 are indicated by the lines drawn around groups of receiveelements 704. As shown, some elements may participate in two or moredifferent receive apertures 704.

As with other embodiments described herein, each receive element may beconnected to a separate channel in a receive subsystem such that echoesreceived by each receive element may be independently stored. Therefore,receive elements may be assigned to apertures after echo data has beencollected. In some cases, receive elements may be assigned to aperturesbased on the known positions of receive elements relative to knownpositions of transmit elements.

In any of the probe embodiments described in this disclosure, pointsource transmitters may take any suitable form, such as a single bulkpiezoelectric element, a segmented piezoelectric element, a coordinatedgroup of bulk piezoelectric elements, or a dedicated transmitter groupof micro-elements which may be operated to transmit spherical waveformpings from an apparent point source at a geometric center of thetransmitter group. Micro-elements making up a transmitter group may bearranged in a planar arrangement relative to one another. Alternately,micro-elements making up a transmitter group of micro-elements may bearranged on a locally concave or convex substrate so as to form anapparent-point-source transducer element.

Sparse Array Probes with Physical Gaps

In various embodiments, transmitter micro-element groups and/or receivermicro-element groups may be made in any other planar, convex, concave,concave and convex, or amorphous shape. For example, micro-elementgroups may be formed in approximately square shapes, approximatelycircular shapes, approximately polygonal shapes, approximatelyconcentric ring shapes, etc. Overall dimensions of transmittermicro-element groups and receiver micro-element groups may be sized asdescribed elsewhere in this disclosure. In some cases, transmittermicro-element groups may be the same size as receiver micro-elementgroups, e.g., with dimensions of between about 150 microns and about 0.5mm or more.

In various embodiments, the use of a sparse array configuration mayallow for other uses of regions in between transducer elements. Forexample, various treatment structures may be provided in spaces betweentransducer elements. Such treatment structures may include highfrequency ultrasound (HIFU) transmitters for delivering targetedablative US treatment, radiation or drug delivery elements, lasers orradio frequency (RF) delivery elements, or any other structures fortreating or diagnosing a patient that may be positioned in spacesbetween transducer elements contributing to a unified image.

In some embodiments, vacuum ports may be provided between transducerelements for causing a tissue or other medium to be drawn into contactwith a surface of the imaging probe. Vacuum ports may comprise holesand/or channels in the substrate, support structures, matching layers,lensing layers, etc.

In some embodiments, sparse array probes may be configured with openingsor gaps through which instruments or tools may be inserted. For example,probes may be configured with gaps sized and configured to receiveinstruments or tools such as scalpels, scrapers, biopsy tools, roboticarms, needles, surgical tools, or other implements.

FIG. 15 illustrates an ultrasound probe made up of a circular array 800of transducer elements 802 with a gap 810 in the center. The circulararray 800 may be made up of array segments 812, 814, 816, 818. Forexample, FIG. 15 shows a probe array made up of four pie-slice segmentseach in the shape of a quarter-circular segment. The segments may berigidly fixed in a housing, bracket, or other structural supportconfigured to hold the segments in a consistent position relative to oneanother. The central gap 810 may be sized to allow insertion of varioustools or instruments as discussed above.

Using ping-based multiple aperture imaging techniques, the regionimmediately under the probe gap 810 may be imaged by transmittingspherical ping signals that insonify the region under the gap, andreceiving echoes with receive elements 802 near the gap 810. In someembodiments, transducer elements closer to the central gap 810 may bespaced more closely to one another so as to provide more receiveelements adjacent to the gap, thereby increasing the number of receiversthat may receive echoes from reflectors lying under the gap 810.

Transmit transducer elements will tend to produce spherical waveformsthat propagate in all directions extending from the point-sourcetransmitter element into the imaged object. At some angle from normal,the strength of the transmitted energy will typically tend to drop offdramatically, thereby defining a threshold angle. Rotating a singleangle about the point-source defines a “signal cone.” Reflectors withinthe signal cone will tend to provide sufficiently high signal-to-noisereflections that they may be reliably imaged, while reflectors outsideof the signal cone may tend to return echoes with too little energy toprovide valuable contributions to the final image. The signal cone forany particular transmit transducer element may be determined empiricallyby experimentation. Receive elements may have a similar signal cone ofpositively-contributing reflectors. As described herein, the angle of atransducer element's signal cone may be related to the size of theelement with smaller elements generally having a wider signal cone thanlarger elements. On the other hand, larger elements may produce moreenergy (or may be sensitive to weaker received signals) than smallerelements.

Based on the size of the gap and the angles of transmitter signal conesand receiver signal cones, transmitter elements and receiver elementsmay be positioned relative to the gap 810 so as to allow effectiveimaging of the volume below the gap 810. For example, in some cases,transmitter elements and receiver elements with wide signal cones (alsoreferred to as “look angles”) may be positioned in a higher densityadjacent to the gap, while transmit and receive elements with narrowersignal cones may be positioned further from the gap. As described invarious examples herein, transmitter elements and receiver elements ofvarious or variable sizes may be used based on the energy requirementsand signal cone shapes.

FIG. 16A and FIG. 16B illustrate another example of an ultrasound probe850 with a physical gap 852 separating array segments 854, 856. Theprobe 850 of FIG. 16A and FIG. 16B is generally configured to providereal-time volumetric imaging of a region below both array segmentsincluding the region below the gap 852 between the segments. The gap 852may be used for performing various surgical, diagnostic, orinterventional procedures.

The probe 850 of FIG. 16A and FIG. 16B may include a bridge handle 860rigidly joining the two array segments 854, 856 and containing conduitsfor electrical connections. The bridge handle may take any shape andstructure as needed in order to rigidly hold the array segments inconsistent positions relative to one another. Based on the size of thegap 852 and the angles of transmitter signal cones and receiver signalcones, transmitter elements and receiver elements may be positionedrelative to the gap 852 so as to allow effective imaging of the volumebelow the gap 852. As above, transmitter and receiver elements 862 ofvarious or variable sizes may be used based on the energy requirementsand signal cone shapes.

Multiple Aperture Ultrasound Imaging System Components

The block diagram of FIG. 17 illustrates components of an ultrasoundimaging system 200 that may be used in combination with variousembodiments of systems and methods as described herein. The system 200of FIG. 17 may include several subsystems: a transmit control subsystem204, a probe subsystem 202, a receive subsystem 210, an image generationsubsystem 230, and a video subsystem 240. In various embodiments, thesystem 200 may also include one or more memory devices for containingvarious data for use during one or more ultrasound imaging steps. Suchmemory devices may include a raw echo data memory 220, a weightingfactor memory 235, a calibration data memory 238, an image buffer 236and/or a video memory 246. In various embodiments all data (includingsoftware and/or firmware code for executing any other process) may bestored on a single memory device. Alternatively, separate memory devicesmay be used for one or more data types.

The transmission of ultrasound signals from elements of the probe 202may be controlled by a transmit control subsystem 204. In someembodiments, the transmit control subsystem 204 may include anycombination of analog and digital components for controlling transducerelements of the probe 202 to transmit un-focused ultrasound pings atdesired frequencies and intervals from selected transmit aperturesaccording to a desired imaging algorithm. In some embodiments a transmitcontrol system 204 may be configured to transmit ultrasound pings at arange of ultrasound frequencies. In some (though not all) embodiments,the transmit control subsystem may also be configured to control theprobe in a phased array mode, transmitting focused (i.e., transmitbeamformed) ultrasound scanline beams.

In some embodiments, a transmit control sub-system 204 may include atransmit signal definition module 206 and a transmit element controlmodule 208. The transmit signal definition module 206 may includesuitable combinations of hardware, firmware and/or software configuredto define desired characteristics of a signal to be transmitted by anultrasound probe. For example, the transmit signal definition module 206may establish (e.g., based on user inputs or on pre-determined factors)characteristics of an ultrasound signal to be transmitted such as apulse start time, pulse length (duration), ultrasound frequency, pulsepower, pulse shape, pulse direction (if any), pulse amplitude, transmitaperture location, or any other characteristics.

The transmit element control module 208 may then take information aboutthe desired transmit pulse and determine the corresponding electricalsignals to be sent to the appropriate transducer elements in order toproduce this signal. In various embodiments, the signal definitionmodule 206 and the transmit element control module 208 may compriseseparate electronic components, or may include portions of one or morecommon components.

Upon receiving echoes of transmitted signals from a region of interest,the probe elements may generate time-varying electrical signalscorresponding to the received ultrasound vibrations. Signalsrepresenting the received echoes may be output from the probe 202 andsent to a receive subsystem 210. In some embodiments, the receivesubsystem may include multiple channels, each of which may include ananalog front-end device (“AFE”) 212 and an analog-to-digital conversiondevice (ADC) 214. In some embodiments, each channel of the receivesubsystem 210 may also include digital filters and data conditioners(not shown) after the ADC 214. In some embodiments, analog filters priorto the ADC 214 may also be provided. The output of each ADC 214 may bedirected into a raw data memory device 220. In some embodiments, anindependent channel of the receive subsystem 210 may be provided foreach receive transducer element of the probe 202. In other embodiments,two or more transducer elements may share a common receive channel.

In some embodiments, an analog front-end device 212 (AFE) may performcertain filtering processes before passing the signal to ananalog-to-digital conversion device 214 (ADC). The ADC 214 may beconfigured to convert received analog signals into a series of digitaldata points at some pre-determined sampling rate. Unlike most ultrasoundsystems, some embodiments of the ultrasound imaging system of FIG. 17may then store digital data representing the timing, phase, magnitudeand/or the frequency of ultrasound echo signals received by eachindividual receive element in a raw data memory device 220 beforeperforming any further receive beamforming, filtering, image layercombining or other image processing.

In order to convert the captured digital samples into an image, the datamay be retrieved from the raw data memory 220 by an image generationsubsystem 230. As shown, the image generation subsystem 230 may includea beamforming block 232 and an image layer combining (“ILC”) block 234.In some embodiments, a beamformer 232 may be in communication with acalibration memory 238 that contains probe calibration data. Probecalibration data may include information about the precise position,operational quality, and/or other information about individual probetransducer elements. The calibration memory 238 may be physicallylocated within the probe, within the imaging system, or in locationexternal to both the probe and the imaging system.

In some embodiments, after passing through the image generation block230, image data may then be stored in an image buffer memory 236 whichmay store beamformed and (in some embodiments) layer-combined imageframes. A video processor 242 within a video subsystem 240 may thenretrieve image frames from the image buffer, and may process the imagesinto a video stream that may be displayed on a video display 244 and/orstored in a video memory 246 as a digital video clip, e.g., as referredto in the art as a “cine loop”.

In some embodiments, the AFE 212 may be configured to perform variousamplification and filtering processes to a received analog signal beforepassing the analog signal to an analog-to-digital conversion device. Forexample, an AFE 212 may include amplifiers such as a low noise amplifier(LNA), a variable gain amplifier (VGA), a bandpass orlowpass/anti-aliasing filter, and/or other amplification or filteringdevices. In some embodiments, an AFE device 212 may be configured tobegin passing an analog signal to an ADC 214 upon receiving a triggersignal. In other embodiments, an AFE device can be “free running”,continuously passing an analog signal to an ADC.

In some embodiments, each analog-to-digital converter 214 may generallyinclude any device configured to sample a received analog signal at someconsistent, predetermined sampling rate. For example, in someembodiments, an analog-to-digital converter may be configured to recorddigital samples of a time-varying analog signal at 25 MHz, which is 25million samples per second or one sample every 40 nanoseconds. Thus,data sampled by an ADC may simply include a list of data points, each ofwhich may correspond to a signal value at a particular instant. In someembodiments, an ADC 214 may be configured to begin digitally sampling ananalog signal upon receiving a trigger signal. In other embodiments, anADC device can be “free running”, continuously sampling a receivedanalog signal.

In some embodiments, the raw data memory device 220 may include anysuitable volatile or non-volatile digital memory storage device. In someembodiments, the raw data memory 220 may also comprise communicationelectronics for transmitting raw digital ultrasound data to an externaldevice over a wired or wireless network. In such cases, the transmittedraw echo data may be stored on the external device in any desiredformat. In other embodiments, the raw data memory 220 may include acombination of volatile memory, non-volatile memory and communicationelectronics.

In some embodiments, the raw data memory device 220 may comprise atemporary (volatile or non-volatile) memory section, and a long-termnon-volatile memory section. In an example of such embodiments, thetemporary memory may act as a buffer between the ADC 214 and thebeamformer 232 in cases where the beamformer 232 may be unable tooperate fast enough to accommodate data at the full rate from the ADC214. In some embodiments, a long-term non-volatile memory device may beconfigured to receive data from a temporary memory device or directlyfrom the ADC 214. Such a long-term memory device may be configured tostore a quantity of raw echo data for subsequent processing, analysis ortransmission to an external device.

In some embodiments, the beamforming block 232 and the image layercombining block 234 may each include any digital signal processingand/or computing components configured to perform the specifiedprocesses (e.g., as described below). For example, in variousembodiments the beamforming 232 and image layer combining 234 may beperformed by software running on a single GPU, on multiple GPUs, on oneor more CPUs, on combinations of CPUs & GPUs, on single or multipleaccelerator cards or modules, on a distributed processing system, or aclustered processing system. Alternatively, these or other processes maybe performed by firmware running on an FPGA (Field Programmable GateArray) architecture or one or more dedicated ASIC (Application-SpecificIntegrated Circuit) devices.

In some embodiments, the video processor 242 may include any videoprocessing hardware, firmware and software components that may beconfigured to assemble image frames into a video stream for displayand/or storage.

In any embodiment, a plurality of elements may share one or moreconductors in a multiplexed arrangement, such astime-division-multiplexed communications, or other multiplexing methods.Multiplexing signals may allow for a cable size to be reduced withoutsacrificing the benefits of individual channels for each element.

In any embodiment, the exact acoustic location of each transmit elementand each receive element may be determined by precision manufacturing,by calibration, or some combination of both. Such element locationinformation may be stored and made available to an image-formationand/or beamforming system.

Certain Terminology

Although this invention has been disclosed in the context of certainpreferred embodiments and examples, it will be understood by thoseskilled in the art that the present invention extends beyond thespecifically disclosed embodiments to other alternative embodimentsand/or uses of the invention and obvious modifications and equivalentsthereof. Various modifications to the above embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, it is intended that the scope ofthe present invention herein disclosed should not be limited by theparticular disclosed embodiments described above, but should bedetermined only by a fair reading of the claims that follow.

In particular, materials and manufacturing techniques may be employed aswithin the level of those with skill in the relevant art. Furthermore,reference to a singular item, includes the possibility that there areplural of the same items present. More specifically, as used herein andin the appended claims, the singular forms “a,” “and,” “said,” and “the”include plural referents unless the context clearly dictates otherwise.Also as used herein, unless explicitly stated otherwise, the term “or”is inclusive of all presented alternatives, and means essentially thesame as the commonly used phrase “and/or.” It is further noted that theclaims may be drafted to exclude any optional element. As such, thisstatement is intended to serve as antecedent basis for use of suchexclusive terminology as “solely,” “only” and the like in connectionwith the recitation of claim elements, or use of a “negative”limitation. Unless defined otherwise herein, all technical andscientific tennis used herein have the same meaning as commonlyunderstood by one of ordinary skill in the art to which this inventionbelongs.

Certain features that are described in this disclosure in the context ofseparate implementations can also be implemented in combination in asingle implementation. Conversely, various features that are describedin the context of a single implementation can also be implemented inmultiple implementations separately or in any suitable subcombination.Although features may be described above as acting in certaincombinations, one or more features from a claimed combination can, insome cases, be excised from the combination, and the combination may beclaimed as any subcombination or variation of any subcombination.Further, the claims may be drafted to exclude any disclosed element. Assuch, the foregoing sentence is intended to serve as antecedent basisfor use of such exclusive terminology as “solely,” “only” and the likein connection with the recitation of claim elements, or use of a“negative” limitation.

Moreover, while operations may be depicted in the drawings or describedin the specification in a particular order, such operations need not beperformed in the particular order shown or in sequential order, and alloperations need not be performed, to achieve the desirable results.Other operations that are not depicted or described can be incorporatedin the example methods and processes. For example, one or moreadditional operations can be performed before, after, simultaneously, orbetween any of the described operations. Further, the operations may berearranged or reordered in other implementations. Also, the separationof various system components in the implementations described aboveshould not be understood as requiring such separation in allimplementations, and it should be understood that the describedcomponents and systems can generally be integrated together in a singleproduct or packaged into multiple products. Additionally, otherimplementations are within the scope of this disclosure.

Some embodiments have been described in connection with the accompanyingdrawings. Some of the figures may be drawn to scale, but such scaleshould not be limiting, since dimensions and proportions other than whatare shown are contemplated and are within the scope of the disclosedinvention. Distances, angles, etc. are merely illustrative and do notnecessarily bear an exact relationship to actual dimensions and layoutof the devices illustrated. Components can be added, removed, and/orrearranged. Further, the disclosure herein of any particular feature,aspect, method, property, characteristic, quality, attribute, element,or the like in connection with various embodiments can be used in allother embodiments set forth herein. Additionally, any methods describedherein may be practiced using any device suitable for performing therecited steps.

What is claimed is:
 1. A ping-based ultrasound transducer probecomprising: an array of ultrasound transducing micro-elements, whereeach micro-element has a diameter less than 500 microns; a first groupof micro-elements electrically connected to a first signal conductor; asecond group of micro-elements electrically connected to a second signalconductor, the second signal conductor being electrically separate fromthe first signal conductor; and a third group of micro-elementspositioned between the first group and the second group, the third groupof micro-elements being permanently disconnected from any signalconductors wherein at least a majority of micro-elements of the arrayare spaced from adjacent micro-elements by a distance of half of amaximum operating wavelength of the array.
 2. The transducer probe ofclaim 1 wherein each micro-element has a diameter between 25 microns and200 microns.
 3. The transducer probe of claim 1 wherein some of themicro-elements of the first group are differently sized than othermicro-elements of the first group, wherein the size of a micro-elementcorresponds its fundamental operating frequency.
 4. The transducer probeof claim 1 wherein the micro-elements of the first group are connectedto a first ground conductor and the micro-elements of the second groupare connected to a second ground conductor not electrically connected tothe first ground conductor.
 5. The transducer probe of claim 1 whereinthe first group of micro-elements includes more micro-elements than thesecond group.
 6. The transducer probe of claim 1 wherein the first groupof micro-elements collectively forms a dedicated transmit element andthe second group of micro-elements collectively forms a dedicatedreceive element.
 7. The transducer probe of claim 1 further comprising afourth group of micro-elements electrically connected to the firstsignal conductor by a switch that, when closed causes the fourth groupto form a combined element with first group.
 8. The transducer probe ofclaim 7, wherein the micro-elements of the fourth group collectivelysurround the micro-elements of the first group.
 9. The transducer probeof claim 7, wherein the fourth group of micro-elements is adjacent tothe first group of micro-elements.
 10. The transducer probe of claim 7,wherein the combined element has a different shape than the first groupalone.
 11. The transducer probe of claim 7, wherein the combined elementhas a shape that is the same as a shape of the first group but adifferent size.